Nickel-zinc battery

ABSTRACT

Provided is a highly reliable nickel-zinc battery including a separator exhibiting hydroxide ion conductivity and water impermeability. The nickel-zinc battery includes a positive electrode containing nickel hydroxide and/or nickel oxyhydroxide; a positive-electrode electrolytic solution in which the positive electrode is immersed, the electrolytic solution containing an alkali metal hydroxide; a negative electrode containing zinc and/or zinc oxide; a negative-electrode electrolytic solution in which the negative electrode is immersed, the electrolytic solution containing an alkali metal hydroxide; a hermetic container accommodating the positive electrode, the positive-electrode electrolytic solution, the negative electrode, and the negative-electrode electrolytic solution; and the separator exhibiting hydroxide ion conductivity and water impermeability and disposed in the hermetic container so as to separate a positive-electrode chamber from a negative-electrode chamber. The alkali metal hydroxide concentration of the positive-electrode electrolytic solution differs from that of the negative-electrode electrolytic solution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of PCT/JP2015/065225filed May 27, 2015, which claims priority to Japanese Patent ApplicationNo. 2014-141710 filed Jul. 9, 2014, the entire contents all of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nickel-zinc battery.

2. Description of the Related Art

Zinc secondary batteries have been developed and studied over manyyears. Unfortunately, these batteries have not yet been put intopractice. This is due to a problem that zinc contained in the negativeelectrode forms dendritic crystals, i.e. dendrites, during a charge modeof the battery and the dendrites break the separator to cause shortcircuit between the negative electrode and the positive electrode. Incontrast, nickel-cadmium batteries and nickel-hydrogen batteries havealready been commercialized. Nickel-zinc secondary batteries, however,have advantages over such commercialized batteries. In specific,nickel-zinc secondary batteries have a very high theoretical density ofcapacity; i.e., about five times that of nickel-cadmium secondarybatteries, 2.5 times that of nickel-hydrogen secondary batteries, and1.3 times that of lithium ion batteries. In addition, nickel-zincsecondary batteries are produced from inexpensive raw materials. Thus, astrong demand has arisen for a technique for preventing the shortcircuit caused by dendritic zinc in zinc secondary batteries.

For example, Patent Document 1 (WO2013/118561) discloses a nickel-zincsecondary battery including a separator composed of ahydroxide-ion-conductive inorganic solid electrolyte between a positiveelectrode and a negative electrode for preventing the short circuitcaused by dendritic zinc, wherein the inorganic solid electrolyte is alayered double hydroxide (LDH) having a basic composition represented bythe formula: M²⁺ _(1−x)M³⁺ _(x)(OH)₂A^(n−) _(x/n).mH₂O (wherein M²⁺represents at least one type of divalent cation, M³⁺ represents at leastone type of trivalent cation, A^(n−) represents an n-valent anion, n isan integer of 1 or more, and x is 0.1 to 0.4).

Sealed nickel-zinc batteries have been disclosed which are provided withnegative electrodes that absorb to recycle oxygen gas generated at theend of a charge mode. For example, Patent Document 2 (JPH05-303978A)discloses a sealed nickel-zinc battery including an electrode assemblyincluding a positive electrode plate, a negative electrode plate, aseparator, and a retainer, and a liquid-retainable layer disposed aroundthe assembly, wherein the liquid-retainable layer is composed of afibrous cellulose material having a length of 0.5 to 50 mm and adiameter of 5 to 100 μm and impregnated with an electrolytic solution.The separator used in the battery disclosed in Patent Document 2 iscomposed of a porous polypropylene membrane treated with a surfactant.Patent Document 3 (JPH06-96795A) discloses a sealed nickel-zinc batteryincluding an electrode assembly, a battery container, and anelectrolytic solution, wherein the negative electrode of the assemblyfaces the bottom of the container, and the electrolytic solution has avolume that is more than 98% and 110% or less of the total spatialvolume of the electrode assembly. The separator used in the battery iscomposed of a microporous film and a cellophane membrane.

A technique has been disclosed for facilitating the permeation of oxygengas generated from a positive electrode through a separator to anegative electrode during an overcharge mode of a battery. For example,Patent Document 4 (JPH05-36394A) discloses a separator for an alkalinebattery, the separator being composed of a porous hydrophobic resinmembrane having a surface coated with at least a hydrophilic fabric.

CITATION LIST Patent Documents

Patent Document 1: WO2013/118561

Patent Document 2: JPH05-303978A

Patent Document 3: JPH06-96795A

Patent Document 4: JPH05-36394A

SUMMARY OF THE INVENTION

The present inventors have found that the use of a separator exhibitinghydroxide ion conductivity and water impermeability can produce a highlyreliable nickel-zinc battery.

An object of the present invention is to provide a highly reliablenickel-zinc battery including a separator exhibiting hydroxide ionconductivity and water impermeability.

An aspect of the present invention provides a nickel-zinc batterycomprising:

-   -   a positive electrode comprising nickel hydroxide and/or nickel        oxyhydroxide;    -   a positive-electrode electrolytic solution comprising an alkali        metal hydroxide, the positive electrode being immersed in the        positive-electrode electrolytic solution;    -   a negative electrode comprising zinc and/or zinc oxide;    -   a negative-electrode electrolytic solution comprising an alkali        metal hydroxide, the negative electrode being immersed in the        negative-electrode electrolytic solution;    -   a hermetic container accommodating the positive electrode, the        positive-electrode electrolytic solution, the negative        electrode, and the negative-electrode electrolytic solution; and    -   a separator exhibiting hydroxide ion conductivity and water        impermeability, the separator being disposed in the hermetic        container so as to separate a positive-electrode chamber        accommodating the positive electrode and the positive-electrode        electrolytic solution from a negative-electrode chamber        accommodating the negative electrode and the negative-electrode        electrolytic solution,    -   wherein the alkali metal hydroxide concentration of the        positive-electrode electrolytic solution differs from the alkali        metal hydroxide concentration of the negative-electrode        electrolytic solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary nickel-zinc batteryaccording to the present invention, the battery being in a discharge endstate.

FIG. 2 illustrates the full charge state of the nickel-zinc battery ofFIG. 1.

FIG. 3 is a schematic illustration of an exemplary parallelly stackednickel-zinc battery according to the present invention, the batterybeing in a discharge end state.

FIG. 4 is a schematic cross-sectional view of a separator provided witha porous substrate in an embodiment.

FIG. 5 is a schematic cross-sectional view of a separator provided witha porous substrate in another embodiment.

FIG. 6 is a schematic illustration of a platy particle of layered doublehydroxide (LDH).

FIG. 7 is a SEM image of the surface of a porous alumina substrateprepared in Example 1.

FIG. 8 is an XRD profile of a crystalline phase of a sample in Example1.

FIG. 9 is a SEM image of a surface microstructure of a sample membranein Example 1.

FIG. 10 is a SEM image of a microstructure at a polished cross-sectionalsurface of a composite material sample in Example 1.

FIG. 11A is an exploded perspective view of a system for evaluating andmeasuring density in Example 1.

FIG. 11B is a schematic cross-sectional view of a system for evaluatingand measuring density in Example 1.

FIG. 12 is a graph illustrating the relationship between KOHconcentration and ionic conductivity in an aqueous KOH solution.

DETAILED DESCRIPTION OF THE INVENTION

Nickel-Zinc Battery

FIG. 1 is a schematic illustration of an exemplary nickel-zinc batteryaccording to the present invention. FIG. 1 illustrates the initial state(i.e., discharge end state) of the nickel-zinc battery before charging.It should be understood that the nickel-zinc battery according to thepresent invention may be in a full charge state. As illustrated in FIG.1, the nickel-zinc battery 10 according to the present inventionincludes a hermetic container 22, and the hermetic container 22 includesa positive electrode 12, a positive-electrode electrolytic solution 14,a negative electrode 16, a negative-electrode electrolytic solution 18,and a ceramic separator 20. The positive electrode 12 contains nickelhydroxide and/or nickel oxyhydroxide. The positive-electrodeelectrolytic solution 14 is an alkaline electrolytic solution containingan alkali metal hydroxide. The positive electrode 12 is immersed in thepositive-electrode electrolytic solution 14. The negative electrode 16contains zinc and/or zinc oxide. The negative-electrode electrolyticsolution 18 contains an alkali metal hydroxide. The negative electrode16 is immersed in the negative-electrode electrolytic solution 18. Thehermetic container 22 accommodates the positive electrode 12, thepositive-electrode electrolytic solution 14, the negative electrode 16,and the negative-electrode electrolytic solution 18. The positiveelectrode 12 is not necessarily separated from the positive-electrodeelectrolytic solution 14, and the positive electrode 12 and thepositive-electrode electrolytic solution 14 may be combined into apositive-electrode mixture. Similarly, the negative electrode 16 is notnecessarily separated from the negative-electrode electrolytic solution18, and the negative electrode 16 and the negative-electrodeelectrolytic solution 18 may be combined into a negative-electrodemixture. A positive-electrode current collector 13 is optionallydisposed in contact with the positive electrode 12, and anegative-electrode current collector 17 is optionally disposed incontact with the negative electrode 16.

The separator 20 is disposed in the hermetic container 22 so as toseparate a positive-electrode chamber 24 accommodating the positiveelectrode 12 and the positive-electrode electrolytic solution 14 from anegative-electrode chamber 26 accommodating the negative electrode 16and the negative-electrode electrolytic solution 18. The separator 20exhibits hydroxide ion conductivity and water impermeability. As usedherein, the term “water impermeability” indicates that water in contactwith one surface of an analyte (e.g., the separator 20 and/or the poroussubstrate 28) does not reach the other surface during the “densityevaluation test” performed in Example 1 described below or any otherequivalent method or system. The water impermeability of the separator20 indicates that the separator 20 has a density sufficiently high toprevent the permeation of water and is not a porous film or porousmaterial having water permeability. Thus, this configuration is veryeffective for physically inhibiting the penetration of dendritic zinc(which may be formed during a charge mode of the battery) through theseparator, to prevent the short circuit between the positive andnegative electrodes. As illustrated in FIG. 1, the separator 20 may beprovided with a porous substrate 28. In any case, the hydroxide ionconductivity of the separator 20 leads to efficient migration ofhydroxide ions between the positive-electrode electrolytic solution 14and the negative-electrode electrolytic solution 18, resulting incharge/discharge reaction in the positive-electrode chamber 24 and thenegative-electrode chamber 26. The following reactions occur at thepositive-electrode chamber 24 and the negative-electrode chamber 26during a charge mode of the battery (reverse reactions occur during adischarge mode).

Ni(OH)₂+OH⁻→NiOOH+H₂O+e ⁻  Positive electrode:

ZnO+H₂O+2e ⁻→Zn+2OH⁻  Negative electrode:

The aforementioned reaction at the negative electrode involves thefollowing two reactions:

ZnO+H₂O+2OH⁻→Zn(OH)₄ ²⁻  Dissolution of ZnO:

Zn(OH)₄ ²⁻+2e ⁻→Zn+4OH⁻  Precipitation of Zn:

In the nickel-zinc battery 10 of the present invention, the alkali metalhydroxide concentration of the positive-electrode electrolytic solution14 differs from that of the negative-electrode electrolytic solution 18.The alkali metal hydroxide concentration of the positive-electrodeelectrolytic solution 14 may incidentally or temporarily equal to thatof the negative-electrode electrolytic solution 18 in association with avariation in alkali metal hydroxide concentration of each electrolyticsolution during the charge/discharge reaction. The present inventionencompasses any nickel-zinc battery so long as the alkali metalhydroxide concentration of the positive-electrode electrolytic solution14 essentially differs from that of the negative-electrode electrolyticsolution 18 throughout the charge/discharge reaction. Most traditionalseparators exhibit water permeability and thus allow water to passtherethrough freely. Thus, the positive-electrode chamber 24 and thenegative-electrode chamber 26 are typically filled with electrolyticsolutions having the same alkali metal hydroxide concentration. Incontrast, the separator 20 used in the present invention has highdensity and water impermeability. Hence, the alkali metal hydroxideconcentrations of the positive-electrode electrolytic solution 14 andthe negative-electrode electrolytic solution 18 can be separatelyoptimized. For example, the alkali metal hydroxide concentration of thepositive-electrode electrolytic solution 14 can be adjusted in view ofan increase in ionic conductivity, whereas the alkali metal hydroxideconcentration of the negative-electrode electrolytic solution 18 can beadjusted in view of an increase in ZnO (discharge product) solubility.Both high ionic conductivity and high ZnO solubility promote thecharge/discharge reaction, resulting in improved charge/discharge ratecharacteristics. In general, the ionic conductivity in an alkalineelectrolytic solution depends on the alkali metal hydroxideconcentration of the electrolytic solution. As illustrated in FIG. 12(i.e., a graph showing the relationship between KOH concentration andionic conductivity in an aqueous KOH solution), the ionic conductivitybecomes maximum at a KOH concentration of about 6 mol/L at 25° C. Anincrease in ZnO solubility promotes the dissolution of ZnO in theaforementioned two-stage reaction at the negative electrode, resultingin promotion of the subsequent precipitation of Zn. In general, anincrease in KOH concentration leads to an increase in ZnO solubility,and KOH is often used at a concentration of about 6 to 9 mol/L. Thus,the alkali metal hydroxide concentrations of the positive-electrodeelectrolytic solution 14 and the negative-electrode electrolyticsolution 18 can be appropriately adjusted in consideration of suchconcentration dependence for suitable control of ionic conductivityand/or ZnO solubility, resulting in improved charge/dischargecharacteristics. The alkali metal hydroxide concentrations can be moredesirably adjusted in consideration that a variation in amount of waterin the positive-electrode chamber 24 significantly differs from that inthe negative-electrode chamber 26 during the charge/discharge mode ofthe battery. As indicated by the aforementioned reaction formulae, theamount of H₂O produced at the positive electrode 12 is twice the amountof H₂O consumed at the negative electrode 16 during a charge mode. Thus,the variation in alkali metal hydroxide concentration of thepositive-electrode electrolytic solution 14 differs from that of thenegative-electrode electrolytic solution 18 (in detail, the variation inalkali metal hydroxide concentration of the positive-electrodeelectrolytic solution 14 is greater than that of the negative-electrodeelectrolytic solution 18) during the charge/discharge mode of thebattery. In the nickel-zinc battery 10 of the present invention, theoptimal alkali metal hydroxide concentration of the positive-electrodeelectrolytic solution 14 can be adjusted separately from that of thenegative-electrode electrolytic solution 18 in consideration of theaforementioned factors, leading to the maximization of the potentials ofthe positive-electrode chamber 24 and the negative-electrode chamber 26,resulting in desired charge/discharge characteristics.

The nickel-zinc battery 10 preferably has an extra positive-electrodespace 25 in the positive-electrode chamber 24. The extrapositive-electrode space 25 has a volume that meets a variation inamount of water in association with the reaction at the positiveelectrode during charge/discharge of the battery. Also, the nickel-zincbattery 10 preferably has an extra negative-electrode space 27 in thenegative-electrode chamber 26. The extra negative-electrode space 27 hasa volume that meets a variation in amount of water in association withthe reaction at the negative electrode during charge/discharge of thebattery. This configuration effectively prevents problems caused by avariation in amount of water in the positive-electrode chamber 24 andthe negative-electrode chamber 26 (e.g., liquid leakage and deformationof the container due to a variation in internal pressure of thecontainer), resulting in further improved reliability of the nickel-zincbattery. As indicated by the aforementioned reaction formulae, theamount of water increases in the positive-electrode chamber 24 anddecreases in the negative-electrode chamber 26 during a charge mode,whereas the amount of water decreases in the positive-electrode chamber24 and increases in the negative-electrode chamber 26 during a dischargemode. Most traditional separators exhibit water permeability and thusallow water to pass therethrough freely. In contrast, the separator 20used in the present invention has high density and water impermeability.Hence, water cannot pass through the separator 20 freely, and anincrease in amount of the electrolytic solution in thepositive-electrode chamber 24 and/or the negative-electrode chamber 26during charge/discharge of the battery may cause problems, such asliquid leakage. As illustrated in FIG. 2, the positive-electrode chamber24 has the extra positive-electrode space 25 having a volume that meetsa variation in amount of water in association with the reaction at thepositive electrode during charge/discharge of the battery, and thus theextra positive-electrode space 25 can buffer an increase in amount ofthe positive-electrode electrolytic solution 14 during a charge mode.Since the extra positive-electrode space 25 serves as a buffer evenafter full charge as illustrated in FIG. 2, an increased amount of thepositive-electrode electrolytic solution 14 can be reliably retained inthe positive-electrode chamber 24 without causing overflow of theelectrolytic solution. Similarly, the negative-electrode chamber 26 hasthe extra negative-electrode space 27 having a volume that meets avariation in amount of water in association with the reaction at thenegative electrode during charge/discharge of the battery, and thus theextra negative-electrode space 27 can buffer an increase in amount ofthe negative-electrode electrolytic solution 18 during a discharge mode.

A variation in amount of water in the positive-electrode chamber 24 orthe negative-electrode chamber 26 can be determined on the basis of theaforementioned reaction formulae. As indicated by the reaction formulae,the amount of H₂O produced at the positive electrode 12 during a chargemode is twice the amount of H₂O consumed at the negative electrode 16.Thus, the volume of the extra positive-electrode space 25 may be greaterthan that of the extra negative-electrode space 27. The volume of theextra positive-electrode space 25 is preferably determined such that thepositive-electrode chamber 24 can be adapted not only to an increasedamount of water, but also to gasses (e.g., air originally contained inthe positive-electrode chamber 24, and oxygen gas generated from thepositive electrode 12 during overcharge) at an appropriate internalpressure (if a gas flow channel 29 is absent). Alternatively, the volumeof the extra positive-electrode space 25 is preferably determined so asto prevent intrusion of an increased amount of the positive-electrodeelectrolytic solution 14 into a gas flow channel 29 (if present).Although the volume of the extra negative-electrode space 27 may beequal to that of the extra positive-electrode space 25 as illustrated inFIG. 1, the volume of the extra negative-electrode space 27 ispreferably greater than the amount of water decreased during a chargemode in the case of the battery in a discharge end state. In any case,the volume of the extra negative-electrode space 27 may be smaller thanthat of the extra positive-electrode space 25 because a variation inamount of water in the negative-electrode chamber 26 is about half thatin the positive-electrode chamber 24.

The nickel-zinc battery 10 in a discharge end state preferably satisfiesthe following conditions: the extra positive-electrode space 25 has avolume greater than the amount of water that will increase inassociation with the reaction at the positive electrode during a chargemode; the extra positive-electrode space 25 is not preliminarily filledwith the positive-electrode electrolytic solution 14; the extranegative-electrode space 27 has a volume greater than the amount ofwater that will decrease in association with the reaction at thenegative electrode during the charge mode; and the extranegative-electrode space 27 is preliminarily filled with an amount ofthe negative-electrode electrolytic solution 18 that will decreaseduring the charge mode. In contrast, the nickel-zinc battery 10 in afull charge state preferably satisfies the following conditions: theextra positive-electrode space 25 has a volume greater than the amountof water that will decrease in association with the reaction at thepositive electrode during a discharge mode; the extra positive-electrodespace 25 is preliminarily filled with an amount of thepositive-electrode electrolytic solution 14 that will decrease duringthe discharge mode; the extra negative-electrode space 27 has a volumegreater than the amount of water that will increase in association withthe reaction at the negative electrode during the discharge mode; andthe extra negative-electrode space 27 is not preliminarily filled withthe negative-electrode electrolytic solution 18.

Preferably, the extra positive-electrode space 25 is not filled with thepositive electrode 12 and/or the extra negative-electrode space 27 isnot filled with the negative electrode 16. More preferably, the extrapositive-electrode space 25 and the extra negative-electrode space 27are not filled with the positive electrode 12 and the negative electrode16, respectively. The electrolytic solution may be depleted due to adecrease in amount of water during charge/discharge of the battery inthese extra spaces. Thus, the positive electrode 12 and the negativeelectrode 16 in these extra spaces are insufficiently involved in thecharge/discharge reaction, resulting in low efficiency. If the extrapositive-electrode space 25 and the extra negative-electrode space 27are not filled with the positive electrode 12 and the negative electrode16, respectively, the positive electrode 12 and the negative electrode16 are effectively and reliably involved in the battery reaction.

The nickel-zinc battery of the present invention preferably has avertical structure having a separator that is vertically disposed. Thevertical disposition of the separator leads to a horizontal arrangementof the positive-electrode chamber, the separator, and thenegative-electrode chamber. If the separator 20 is vertically disposedas illustrated in FIG. 1, the positive-electrode chamber 24 typicallyhas an extra positive-electrode space 25 in its upper portion, and thenegative-electrode chamber 26 has an extra negative-electrode space 27in its upper portion. If the electrolytic solution is in the form ofgel, the electrolytic solution can be retained in a charge/dischargereaction region of the positive-electrode chamber 24 and/or thenegative-electrode chamber 26 despite a reduction in amount of theelectrolytic solution. Thus, the extra positive-electrode space 25and/or the extra negative-electrode space 27 may be provided in anyportion other than the upper portion (e.g., a lateral or lower portion)of the positive-electrode chamber 24 and/or in any portion other thanthe upper portion (e.g., a lateral or lower portion) of thenegative-electrode chamber 26, respectively, resulting in a high designfreedom.

Alternatively, the nickel-zinc battery of the present invention may havea horizontal structure having a separator that is horizontally disposed.The horizontal disposition of the separator leads to a verticalarrangement of the positive-electrode chamber, the separator, and thenegative-electrode chamber. If the electrolytic solution is in the formof gel, the electrolytic solution is always in contact with theseparator despite a reduction in amount of the electrolytic solution. Asecond separator (resin separator) composed of a hygroscopic resin or aliquid-retainable resin (e.g., non-woven fabric) may be disposed betweenthe positive electrode and the separator and/or between the negativeelectrode and the separator such that the electrolytic solution can beretained in a charge/discharge reaction portion of the positiveelectrode and/or the negative electrode despite a reduction in amount ofthe electrolytic solution. Preferred examples of the hygroscopic resinor the liquid-retainable resin include polyolefin resins. Thus, theextra positive-electrode space and/or the extra negative-electrode spacemay be provided in any portion other than the upper portion (e.g., alateral or lower portion) of the positive-electrode chamber and/or inany portion other than the upper portion (a lateral or lower portion) ofthe negative-electrode chamber, respectively.

Separator

The separator 20 exhibits hydroxide ion conductivity and waterimpermeability, and is typically in a plate, membrane, or layer form.The separator 20 is disposed in the hermetic container 22 so as toseparate the positive-electrode chamber 24 accommodating the positiveelectrode 12 and the positive-electrode electrolytic solution 14 fromthe negative-electrode chamber 26 accommodating the negative electrode16 and the negative-electrode electrolytic solution 18.

The separator 20 is preferably composed of an inorganic solidelectrolyte exhibiting hydroxide ion conductivity. The use of theseparator composed of a hydroxide-ion-conductive inorganic solidelectrolyte as the separator 20 separates the electrolytic solutionsbetween the positive and negative electrodes, and ensures conduction ofhydroxide ions. The inorganic solid electrolyte constituting theseparator 20 is typically a dense and hard inorganic solid electrolyte,and thus can physically inhibits the penetration of dendritic zinc(which may be formed during a charge mode of the battery) through theseparator, to prevent the short circuit between the positive andnegative electrodes, resulting in significantly improved reliability ofthe nickel-zinc battery. The inorganic solid electrolyte is desirablydensified to exhibit water impermeability. For example, the inorganicsolid electrolyte has a relative density of preferably 90% or more, morepreferably 92% or more, still more preferably 95% or more, as determinedby the Archimedes method. The density may be any value so long as theinorganic solid electrolyte is dense and hard enough to prevent thepenetration of dendritic zinc. Such a dense and hard inorganic solidelectrolyte may be produced through hydrothermal treatment. Thus, agreen compact which has not undergone hydrothermal treatment is notsuitable as the inorganic solid electrolyte in the present inventionbecause the compact is not dense but brittle in the solution. Anyprocess other than hydrothermal treatment may be used for producing adense and hard inorganic solid electrolyte.

The separator 20 or the inorganic solid electrolyte may be in the formof a composite body containing particles of an organic solid electrolyteexhibiting hydroxide ion conductivity and an auxiliary component thatpromotes the densification or hardening of the particles. Alternatively,the separator 20 may be in the form of a composite body containing aporous body serving as a substrate and an inorganic solid electrolyte(e.g., a layered double hydroxide) that is precipitated and grown inpores of the porous body. Examples of the materials of the porous bodyinclude ceramic materials, such as alumina and zirconia; and insulatingmaterials, such as porous sheets composed of foamed resin or fibrousmaterial.

The inorganic solid electrolyte preferably contains a layered doublehydroxide (LDH) having a basic composition represented by the formula:M²⁺ _(1−x)M³⁺ _(x)(OH)₂A^(n−) _(x/n).mH₂O (wherein M²⁺ represents adivalent cation, M³⁺ represents a trivalent cation, A^(n−) represents ann-valent anion, n is an integer of 1 or more, x is 0.1 to 0.4, and m isany real number). The inorganic solid electrolyte is more preferablycomposed of such an LDH. In the formula, M²⁺ may represent any divalentcation, and is preferably Mg²⁺, Ca²⁺ or Zn²⁺, more preferably Mg²⁺. M³⁺may represent any trivalent cation, and is preferably Al³⁺ or Cr³⁺, morepreferably Al³⁺. A^(n−) may represent any anion, and is preferably OH⁻or CO₃ ²⁻. In the formula, preferably, M²⁺ comprises Mg²⁺, M³⁺ comprisesAl³⁺, and A^(n−) comprises OH⁻ and/or CO₃ ²⁻. In the formula, n is aninteger of 1 or more, preferably 1 or 2; x is 0.1 to 0.4, preferably 0.2to 0.35; and m is any real number. Specifically, m is 0 or more,typically a real or integer number exceeding 0 or not less than 1. Inthe formula, M³⁺ may be partially or entirely replaced with a cationhaving a valency of 4 or more. In such a case, the coefficient x/n ofthe anion A^(n−) in the formula may be appropriately varied.

The inorganic solid electrolyte is preferably densified throughhydrothermal treatment. The hydrothermal treatment is very effective forthe densification of a layered double hydroxide, in particular, an Mg—Allayered double hydroxide. The densification by the hydrothermaltreatment involves, for example, a process described in Patent Document1 (WO2013/118561), in which pure water and a green compact plate treatedin a pressure container at a temperature of 120 to 250° C., preferably180 to 250° C. for 2 to 24 hours, preferably 3 to 10 hours. A morepreferred process involving the hydrothermal treatment will be describedbelow.

The inorganic solid electrolyte may be in a plate, membrane, or layerform. The inorganic solid electrolyte in a membrane or layer form ispreferably disposed on or in the porous substrate. The inorganic solidelectrolyte in the form of a plate has a sufficient hardness andeffectively prevents the penetration of dendritic zinc. The inorganicsolid electrolyte in a membrane or layer form having a thickness smallerthan that of the plate is advantageous in that the electrolyte has aminimum hardness required for preventing the penetration of dendriticzinc and significantly reduces the resistance of the separator. Theinorganic solid electrolyte in the form of a plate has a thickness ofpreferably 0.01 to 0.5 mm, more preferably 0.02 to 0.2 mm, still morepreferably 0.05 to 0.1 mm. The inorganic solid electrolyte preferablyexhibits a high hydroxide ion conductivity. The inorganic solidelectrolyte typically exhibits a hydroxide ion conductivity of 10⁻⁴ to10⁻¹ S/m. The inorganic solid electrolyte in a membrane or layer formhas a thickness of preferably 100 μm or less, more preferably 75 μm orless, still more preferably 50 μm or less, particularly preferably 25 μmor less, most preferably 5 μm or less. Such a small thickness achieves areduction in resistance of the separator 20. The lower limit of thethickness may vary depending on the intended use of the inorganic solidelectrolyte. The thickness is preferably 1 μm or more, more preferably 2μm or more in order to secure a hardness required for a separatormembrane or layer.

A porous substrate 28 may be disposed on either or both of the surfacesof the separator 20. The porous substrate 28 has water permeability, andthus the positive-electrode electrolytic solution 14 and thenegative-electrode electrolytic solution 18 permeate the substrate andreach the separator. The presence of the porous substrate 28 leads toreliable retention of hydroxide ions on the separator 20. The strengthimparted by the porous substrate 28 can reduce the thickness of theseparator 20, resulting in a reduction in resistance. A dense membraneor layer of the inorganic solid electrolyte (preferably LDH) may beformed on or in the porous substrate 28. The disposition of the poroussubstrate on one surface of the separator 20 probably involves a processincluding preparation of the porous substrate 28 and formation of amembrane of the inorganic solid electrolyte on the porous substrate(this process will be described below). In contrast, the disposition ofthe porous substrate on the two surfaces of the separator 20 probablyinvolves a process including densification of the raw powder of theinorganic solid electrolyte disposed between two porous substrates. Withreference to FIG. 1, the porous substrate 28 is disposed entirely on onesurface of the separator 20. Alternatively, the porous substrate 28 maybe disposed only on a portion (e.g., a region responsible forcharge/discharge reaction) of one surface of the separator 20. Forexample, the formation of a membrane or layer of the inorganic solidelectrolyte on or in the porous substrate 28 typically leads to theprocess-derived structure; i.e., the porous substrate is disposedentirely on one surface of the separator 20. In contrast, the formationof an independent plate of the inorganic solid electrolyte (having nosubstrate) may involve the subsequent step of disposing the poroussubstrate 28 on a portion (e.g., a region responsible forcharge/discharge reaction) or the entirety of one surface of theseparator 20.

As described above, a second separator (resin separator) composed of ahygroscopic resin or a liquid-retaining resin (e.g., non-woven fabric)may be disposed between the positive electrode 12 and the separator 20and/or between the negative electrode 16 and the separator 20 such thatthe electrolytic solution can be retained in a reaction portion of thepositive electrode and/or the negative electrode despite a reduction inamount of the electrolytic solution. Preferred examples of thehygroscopic resin or the liquid-retaining resin include polyolefinresins.

Positive Electrode

The positive electrode 12 contains nickel hydroxide and/or nickeloxyhydroxide. The nickel-zinc battery in a discharge end stateillustrated in FIG. 1 may involve the use of nickel hydroxide in thepositive electrode 12. The nickel-zinc battery in a full charge stateillustrated in FIG. 2 may involve the use of nickel oxyhydroxide in thepositive electrode 12. Nickel hydroxide or nickel oxyhydroxide is acommon positive-electrode active material used in nickel-zinc batteriesand is typically in a particulate form. Nickel hydroxide or nickeloxyhydroxide may form a solid solution in the crystal lattice with anelement other than nickel for an improvement in charge efficiency athigh temperature. Examples of the element include zinc and cobalt.Nickel hydroxide or nickel oxyhydroxide may be mixed with a cobaltcomponent. Examples of the cobalt component include particulate metalliccobalt and particulate cobalt oxide (e.g., cobalt monoxide). Particulatenickel hydroxide or nickel oxyhydroxide (which may form a solid solutionwith an element other than nickel) may be coated with a cobalt compound.Examples of the cobalt compound include cobalt monoxide, α-cobalt (II)hydroxide, β-cobalt (II) hydroxide, cobalt compounds having a valency ofmore than 2, and any combination thereof.

The positive electrode 12 may contain an additional element besides thenickel hydroxide compound and the element that may form a solid solutionwith the compound. Examples of the additional element include scandium(Sc), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd),promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), lutetium(Lu), hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium(Os), iridium (Ir), platinum (Pt), gold (Au), mercury (Hg), and anycombination thereof. Such an additional element may be contained in anyform, such as elemental metal or a metal compound (e.g., oxide,hydroxide, halide, or carbonate). The amount of the additional element(in the form of elemental metal or metal compound) is preferably 0.5 to20 parts by weight, more preferably 2 to 5 parts by weight, relative to100 parts by weight of the nickel hydroxide compound.

The positive electrode 12 may be combined with the electrolytic solutionto form a positive-electrode mixture. The positive-electrode mixture maycontain the particulate nickel hydroxide compound, the electrolyticsolution, and optionally an electrically conductive material (e.g.,particulate carbon) or a binder.

The positive-electrode current collector 13 is preferably disposed incontact with the positive electrode 12. As illustrated in FIG. 1, thepositive-electrode current collector 13 may extend to the outside of thehermetic container 22 to serve as a positive-electrode terminal.Alternatively, the positive-electrode current collector 13 may beconnected to a separately provided positive-electrode terminal inside oroutside of the hermetic container 22. Preferred examples of thepositive-electrode current collector 13 include nickel poroussubstrates, such as foamed nickel plates. In such a case, a pastecontaining an electrode active material (e.g., nickel hydroxide) may beevenly applied onto a nickel porous substrate and then dried, to preparea positive electrode plate composed of the positive electrode 12 on thepositive-electrode current collector 13. After the drying step, thepositive electrode plate (i.e., the positive electrode 12 on thepositive-electrode current collector 13) is preferably subjected topressing for prevention of detachment of the electrode active materialor an improvement in electrode density.

Negative Electrode

The negative electrode 16 contains zinc and/or zinc oxide. Zinc may becontained in any form exhibiting electrochemical activity suitable forthe negative electrode; for example, in the form of metallic zinc, azinc compound, or a zinc alloy. Preferred examples of the negativeelectrode material include zinc oxide, metallic zinc, and calciumzincate. More preferred is a mixture of metallic zinc and zinc oxide.The negative electrode 16 may be in the form of gel, or may be combinedwith the electrolytic solution to form a negative-electrode mixture. Forexample, the negative electrode in the form of gel may be readilyprepared through addition of the electrolytic solution and a thickenerto the negative-electrode active material. Examples of the thickenerinclude poly(vinyl alcohol), poly(acrylic acid) salts, CMC, and alginicacid. Preferred is poly(acrylic acid), which exhibits high resistance toa strong alkali.

The zinc alloy may be a non-amalgamated zinc alloy; i.e., a zinc alloynot containing mercury or lead. For example, a zinc alloy containing0.01 to 0.06 mass % indium, 0.005 to 0.02 mass % bismuth, and 0.0035 to0.015 mass % aluminum is preferred because of the effect of reducing thegeneration of hydrogen gas. In particular, indium and bismuth areadvantageous in improving discharge performance. The use of a zinc alloyin the negative electrode retards the self-dissolution in the alkalineelectrolytic solution, to reduce the generation of hydrogen gas,resulting in improved safety.

The negative electrode material may be in any form, but is preferably ina powdery form. The powdery negative electrode material has a largesurface area and is adapted to large current discharge. The negativeelectrode material (in the case of a zinc alloy) preferably has a meanparticle size of 90 to 210 The negative electrode material having such amean particle size has a large surface area and thus is adapted to largecurrent discharge. In addition, the negative electrode material can beevenly mixed with the electrolytic solution or a gelling agent, and isreadily handled during the assembly of the battery.

The negative-electrode current collector 17 is preferably disposed incontact with the negative electrode 16. As illustrated in FIG. 1, thenegative-electrode current collector 17 may extend to the outside of thehermetic container 22 to serve as a negative-electrode terminal.Alternatively, the negative-electrode current collector 17 may beconnected to a separately provided negative-electrode terminal inside oroutside of the hermetic container 22. Preferred examples of thenegative-electrode current collector 17 include punched copper sheets.In such a case, a mixture containing zinc oxide powder and/or zincpowder and an optional binder (e.g., particulatepolytetrafluoroethylene) may be applied onto a punched copper sheet toprepare a negative electrode plate composed of the negative electrode 16on the negative-electrode current collector 17. After the drying of themixture, the negative electrode plate (i.e., the negative electrode 16on the negative-electrode current collector 17) is preferably subjectedto pressing for prevention of detachment of the electrode activematerial or an improvement in electrode density.

Electrolytic Solution

Each of the positive-electrode electrolytic solution 14 and thenegative-electrode electrolytic solution 18 contains an alkali metalhydroxide. Specifically, each of the positive-electrode electrolyticsolution 14 and the negative-electrode electrolytic solution 18illustrated in FIG. 1 is an aqueous alkali metal hydroxide solution.Examples of the alkali metal hydroxide include potassium hydroxide,sodium hydroxide, lithium hydroxide, and ammonium hydroxide. Morepreferred is potassium hydroxide. The electrolytic solution may containa zinc compound, such as zinc oxide or zinc hydroxide, for preventingthe self-dissolution of a zinc alloy. As described above, thepositive-electrode electrolytic solution 14 and the negative-electrodeelectrolytic solution 18 may be in the form of a positive-electrodemixture and/or a negative-electrode mixture prepared through combinationwith the positive electrode 12 and/or the negative electrode 16.Alternatively, the alkaline electrolytic solution may be formed into agel for preventing the leakage of the solution. The gelling agent ispreferably a polymer that swells through absorption of the solvent ofthe electrolytic solution. Examples of the gelling agent includepolymers, such as poly(ethylene oxide), poly(vinyl alcohol), andpolyacrylamide; and starch.

In the nickel-zinc battery 10 of the present invention, thepositive-electrode electrolytic solution 14 is not in communication withthe negative-electrode electrolytic solution 18. Thus, thepositive-electrode electrolytic solution 14 may have a compositiondifferent from that of the negative-electrode electrolytic solution 18.Particularly preferably, the positive-electrode electrolytic solution 14is an aqueous potassium hydroxide solution, and the negative-electrodeelectrolytic solution 18 is an aqueous potassium hydroxide solutioncontaining Zn(OH)₄ ²⁻. As described above, the following two reactionsoccur at the negative electrode:

ZnO+H₂O+2OH→Zn(OH)₄ ²⁻  Dissolution of ZnO:

Zn(OH)₄ ²⁻+2e ⁻→Zn+4OH⁻  Precipitation of Zn:

The preliminary preparation of an aqueous potassium hydroxide solutioncontaining Zn(OH)₄ ²⁻ (preferably in a saturated state) by thedissolution of ZnO can promote the precipitation of Zn in theaforementioned two-stage reaction, resulting in improvedcharge/discharge rate characteristics.

As described above, the alkali metal hydroxide concentration of thepositive-electrode electrolytic solution 14 differs from that of thenegative-electrode electrolytic solution 18. The alkali metal hydroxideconcentration of the positive-electrode electrolytic solution ispreferably 1 to 9 mol/L, more preferably 3 to 9 mol/L, still morepreferably 3 to 8 mol/L, particularly preferably 4 to 8 mol/L. Asindicated by the graph of FIG. 12 (i.e., the relationship between KOHconcentration and ionic conductivity in an aqueous KOH solution), such apreferred range of concentration leads to an increase in ionicconductivity. The alkali metal hydroxide concentration of thenegative-electrode electrolytic solution is preferably 3 to 18 mol/L,more preferably 5 to 15 mol/L, still more preferably 6 to 10 mol/L,particularly preferably 6 to 7.5 mol/L. Such a preferred range ofconcentration leads to an increase in ZnO solubility. High ZnOsolubility demands that the alkali metal hydroxide concentration of thenegative-electrode electrolytic solution 18 be generally higher thanthat of the positive-electrode electrolytic solution 14. Thus, theaverage alkali metal hydroxide concentration of the negative-electrodeelectrolytic solution 18 is preferably higher than that of thepositive-electrode electrolytic solution 14 from a full charge state toa discharge end state or vice versa. Both high ionic conductivity andhigh ZnO solubility promote the charge/discharge reaction, resulting inimproved charge/discharge rate characteristics.

In a full charge state, the alkali metal hydroxide concentration of thepositive-electrode electrolytic solution 14 is preferably lower thanthat of the negative-electrode electrolytic solution 18. Dischargingfrom a full charge state leads to consumption of H₂O (a reduction inamount of water) at the positive electrode 12, resulting in an increasein alkali metal hydroxide concentration of the positive-electrodeelectrolytic solution 14. Thus, a relatively low alkali metal hydroxideconcentration of the positive-electrode electrolytic solution 14 in afull charge state leads to a variation in alkali metal hydroxideconcentration within a desired range until a discharge end state. In adischarge end state, the alkali metal hydroxide concentration of thepositive-electrode electrolytic solution 14 is preferably higher thanthat of the negative-electrode electrolytic solution 18. Charging from adischarge end state leads to production of H₂O (an increase in amount ofwater) at the positive electrode 12, resulting in a reduction in alkalimetal hydroxide concentration of the positive-electrode electrolyticsolution 14. Thus, a relatively high alkali metal hydroxideconcentration of the positive-electrode electrolytic solution 14 in adischarge end state leads to a variation in alkali metal hydroxideconcentration within a desired range until a full charge state.

The nickel-zinc battery 10 may be prepared in a discharge end state. Insuch a case, the alkali metal hydroxide concentration of thepositive-electrode electrolytic solution 14 is preferably higher thanthat of the negative-electrode electrolytic solution 18 during thepreparation of the battery. Charging from a discharge end state leads toproduction of H₂O (an increase in amount of water) at the positiveelectrode 12, resulting in a reduction in alkali metal hydroxideconcentration of the positive-electrode electrolytic solution 14. Thus,a relatively high alkali metal hydroxide concentration of thepositive-electrode electrolytic solution 14 in a discharge end stateleads to a variation in alkali metal hydroxide concentration within adesired range until a full charge state. The nickel-zinc battery 10 maybe prepared in a full charge state. In such a case, the alkali metalhydroxide concentration of the positive-electrode electrolytic solution14 is preferably lower than that of the negative-electrode electrolyticsolution 18 during the preparation of the battery.

Discharging from a full charge state leads to consumption of H₂O (areduction in amount of water) at the positive electrode 12, resulting inan increase in alkali metal hydroxide concentration of thepositive-electrode electrolytic solution 14. Thus, a relatively lowalkali metal hydroxide concentration of the positive-electrodeelectrolytic solution 14 in a full charge state leads to a variation inalkali metal hydroxide concentration within a desired range until adischarge end state.

Hermetic Container

The hermetic container 22 accommodates the positive electrode 12, thepositive-electrode electrolytic solution 14, the negative electrode 16,and the negative-electrode electrolytic solution 18, and has a structureexhibiting liquid and gas tightness. The hermetic container may becomposed of any material exhibiting resistance to an alkali metalhydroxide, such as potassium hydroxide. The material is preferably aresin, such as a polyolefin resin, an ABS resin, or a modifiedpolyphenylene ether, more preferably an ABS resin or a modifiedpolyphenylene ether. The separator 20 may be fixed to the hermeticcontainer 22 by any known technique, preferably with an adhesiveexhibiting resistance to an alkali metal hydroxide, such as potassiumhydroxide. It is also preferred that the separator 20 be fixed bythermal welding to the hermetic container 22 composed of a polyolefinresin.

Gas Flow Channel

Preferably, the nickel-zinc battery 10 further includes a gas flowchannel 29 that connects the extra positive-electrode space 25 to theextra negative-electrode space 27 such that these spaces are in gascommunication with each other. The separator 20 used in the presentinvention has a density sufficiently high to prevent the permeation ofwater; i.e., the separator is composed of a material having no or verylow gas permeability. Through provision of the gas flow channel 29,gases (e.g., air originally contained in the positive-electrode chamber24 or the negative-electrode chamber 26) can be accommodated in theentire hermetic container 22 including the extra positive-electrodespace 25 and the extra negative-electrode space 27, rather than beingaccommodated only in the extra positive-electrode space 25 or the extranegative-electrode space 27. This configuration contributes not only tospace saving, but also to a significant improvement in overchargeresistance. In the nickel-zinc battery, oxygen gas may be generated fromthe positive electrode 12 during an overcharge mode (refer to, forexample, Patent Document 4 (JPH05-36394A)). The oxygen gas can betransferred to the negative-electrode chamber 26 through the gas flowchannel 29 and then absorbed by the negative electrode 16 or recombinedwith hydrogen. In particular, the oxygen gas generated from the positiveelectrode 12 may cause an imbalance in volume between the positiveelectrode 12 and the negative electrode 16, resulting in generation ofhydrogen gas from the negative electrode 16. The aforementioned problemscan be prevented or reduced by incorporation of the oxygen gas into thebattery system through absorption of the oxygen gas by the negativeelectrode 16 or recombination of the oxygen gas with hydrogen gas. Thus,the overcharge resistance can be significantly improved. Althoughhydrogen gas undesirably promotes the precipitation of dendritic zinc,the aforementioned improvement in overcharge resistance by the gas flowchannel 29 probably contributes to a reduction in precipitation ofdendritic zinc. The loss of water can be reduced in thepositive-electrode electrolytic solution 14 and the negative-electrodeelectrolytic solution 18 by prevention of unlimited generation of oxygenand hydrogen gases.

The gas flow channel 29 may have any known structure that connects theextra positive-electrode space 25 to the extra negative-electrode space27 such that these spaces are in gas communication with each other. Thegas flow channel 29 is preferably a bypass tube. For example, the gasflow channel 29 illustrated in FIG. 1 is in the form of a bypass tubedisposed outside the hermetic container 22. One end of the bypass tubeis connected to the extra positive-electrode space 25 through thehermetic container 22 so as to be in gas communication with the space25, whereas the other end of the bypass tube is connected to the extranegative-electrode space 27 through the hermetic container 22 so as tobe in gas communication with the space 27. The bypass tube may bemodified (e.g., thinned) and disposed inside the hermetic container 22or in the wall of the hermetic container 22. The gas flow channel 29 isalso preferably a gap provided between the separator 20 and the hermeticcontainer 22. For example, the battery may be configured such that theupper end of the separator 20 is slightly away from the inner wall ofthe hermetic container 22 and the extra positive-electrode space 25 isin gas communication with the extra negative-electrode space 27 via agap between the separator 20 and the hermetic container 22. Forprovision of such a gap, the size and position of the separator 20 maybe appropriately adjusted, or the lid of the hermetic container 22 maybe fitted to the body of the hermetic container 22 via positioning means(e.g., a spacer). Alternatively, a dent may be formed at a predeterminedposition of the inner wall of the hermetic container 22 such that a gapis provided between the hermetic container 22 and the separator 20.

The gas flow channel 29 is preferably disposed so as to establish aconnection between a portion of the positive-electrode chamber 24located at a position which the positive-electrode electrolytic solution14 does not reach even after an increase in amount of water throughcharging and a portion of the negative-electrode chamber 26 located at aposition which the negative-electrode electrolytic solution 18 does notreach even after an increase in amount of water through discharging. Inthis case, reliable gas communication between the spaces can be ensuredat any stage of the charge/discharge reaction. In the case where theseparator 20 is vertically disposed, the extra positive-electrode space25 is provided in an upper portion of the positive-electrode chamber 24,and the extra negative-electrode space 27 is provided in an upperportion of the negative-electrode chamber 26 as illustrated in FIG. 1,the gas flow channel 29 is preferably disposed so as to connect the top(or the vicinity thereof) of the positive-electrode chamber 24 to thetop (or the vicinity thereof) of the negative-electrode chamber 26 forthe following reason. Since the top (or the vicinity thereof) of thepositive-electrode chamber 24 or the negative-electrode chamber 26 islocated at an upper portion of the extra positive-electrode space 25 orthe extra negative-electrode space 27, respectively, thepositive-electrode electrolytic solution 14 or the negative-electrodeelectrolytic solution 18 cannot reach the top (or the vicinity thereof)of the positive-electrode chamber 24 or the negative-electrode chamber26, respectively during the common operation of the battery. Asdescribed above, the extra positive-electrode space 25 and/or the extranegative-electrode space 27 may be provided in any portion other thanthe upper portion (e.g., a lateral or lower portion) of thepositive-electrode chamber 24 and/or in any portion other than the upperportion (e.g., a lateral or lower portion) of the negative-electrodechamber 26, respectively. Thus, the position which the electrolyticsolution does not reach is not necessarily located at the top (or thevicinity thereof) of the positive-electrode chamber 24 or thenegative-electrode chamber 26.

In any case, the gas flow channel 29 is preferably disposed such thatneither the positive-electrode electrolytic solution 14 nor thenegative-electrode electrolytic solution 18 passes therethrough.Although the gas flow channel 29 is basically disposed at a positionwhich the electrolytic solution does not reach as described above, thegas flow channel 29 may have any structure capable of reliablypreventing the intrusion or passage of the electrolytic solution. Forexample, the gas flow channel 29 may be provided with a gas-permeablewater-repellent membrane at any position for preventing the incidentalintrusion or passage of water. If the gas flow channel 29 is a bypasstube, the aforementioned water-repellent membrane may be disposed at anyposition in the bypass tube and/or on either or both of the ends of thebypass tube. If the gas flow channel 29 is a gap provided between theseparator 20 and the hermetic container 22, the aforementionedwater-repellent membrane may be disposed to cover the gap. Such aconfiguration can prevent mixing of the positive-electrode electrolyticsolution 14 with the negative-electrode electrolytic solution 18 in thebattery prepared not to be turned over (see FIG. 1) even if the batteryundergoes an unexpected movement or arrangement, such as vibration,inclination, or turnover.

Parallelly Stacked Nickel-Zinc Battery

The nickel-zinc battery 10 illustrated in FIG. 1 includes one pair ofthe positive electrode 12 and the negative electrode 16. The nickel-zincbattery may include two or more pairs of the positive electrode 12 andthe negative electrode 16 disposed in the hermetic container 22.Preferably, positive electrodes 12 and negative electrodes 16 arealternately disposed to form a parallelly stacked nickel-zinc battery.FIG. 3 illustrates an exemplary parallelly stacked nickel-zinc battery.The parallelly stacked nickel-zinc battery 30 of FIG. 3 includes, insequence, a first positive-electrode chamber 24 a (including a positiveelectrode 12 disposed on one surface of a positive-electrode collector13); a separator 20; a first negative-electrode chamber 26 a (includingnegative electrodes 16 disposed on the two surfaces of anegative-electrode collector 17); a separator 20; a secondpositive-electrode chamber 24 b (including positive electrodes 12disposed on the two surfaces of a positive-electrode collector 13); aseparator 20; a second negative-electrode chamber 26 b (includingnegative electrodes 16 disposed on the two surfaces of anegative-electrode collector 17); a separator 20; and a thirdpositive-electrode chamber 24 c (including a positive electrode 12disposed on one surface of a positive-electrode collector 13). Adjacentpositive-electrode and negative-electrode chambers are connected by aflow channel 29 such that these chambers are in gas communication witheach other. With reference to FIG. 3, the components in each of thepositive-electrode chambers 24 a, 24 b, and 24 c are the same as thosein the positive-electrode chamber 24 illustrated in FIG. 1, and thesecomponents are denoted by the same reference numerals as in FIG. 1.Similarly, the components in each of the negative-electrode chambers 26a and 26 b are the same as those in the negative-electrode chamber 26illustrated in FIG. 1, and these components are denoted by the samereference numerals as in FIG. 1. Thus, appropriate arrangement of apredetermined number of repeating assemblies (each including apositive-electrode chamber, a separator, and a negative-electrodechamber in sequence) can produce a parallelly stacked nickel-zincbattery including a predetermined number of positive and negativeelectrodes.

LDH Separator with Porous Substrate

In the present invention, the inorganic solid electrode of the separatormay be in a membrane or layer form as described above. Preferably, theinorganic solid electrode in a membrane or layer form is disposed on orin a porous substrate, to prepare a separator provided with the poroussubstrate. The particularly preferred separator provided with the poroussubstrate includes a porous substrate and a separator layer formed onand/or in the porous substrate. The separator layer contains theaforementioned layered double hydroxide (LDH). The separator layerpreferably exhibits water impermeability. The porous substrate exhibitswater permeability because of the presence of pores, while the separatorlayer composed of LDH is densified to exhibit water impermeability. Theseparator layer is preferably formed on the porous substrate. Asillustrated in FIG. 4, it is preferred that the separator layer 20 inthe form of an LDH dense membrane be formed on the porous substrate 28.In view of the characteristics of the porous substrate 28, LDH particlesmay be formed in pores in the surface and its vicinity as illustrated inFIG. 4. Alternatively, as illustrated in FIG. 5, LDH may be denselyformed in the porous substrate 28 (e.g., in pores in the surface and itsvicinity of the porous substrate 28) such that at least a portion of theporous substrate 28 forms the separator layer 20′. The separatorillustrated in FIG. 5 has a structure prepared by removal of a portioncorresponding to the membrane of the separator layer 20 of the separatorillustrated in FIG. 4. The separator may have any other structure suchthat the separator layer is disposed parallel to the surface of theporous substrate 28. In any case, the separator layer composed of LDH ishighly-densified and thus exhibits water impermeability. Thus, theseparator layer exhibits particular characteristics, i.e. hydroxide ionconductivity and water impermeability.

The porous substrate is preferably one on which and/or in which theLDH-containing separator layer can be formed. The porous substrate maybe composed of any material and may have any porous structure. In atypical embodiment, the LDH-containing separator layer is formed onand/or in the porous substrate. Alternatively, the LDH-containingseparator layer may be formed on a non-porous substrate, and then thenon-porous substrate may be modified into a porous form by any knownprocess. The porous substrate preferably has a water-permeable porousstructure because such a porous structure enables an electrolyticsolution to come into contact with the separator layer in the case ofthe use of the layer as a separator for a battery.

The porous substrate is preferably composed of at least one selectedfrom the group consisting of ceramic materials, metal materials, andpolymer materials. The porous substrate is more preferably composed of aceramic material. Preferred examples of the ceramic material includealumina, zirconia, titania, magnesia, spinel, calcia, cordierite,zeolite, mullite, ferrite, zinc oxide, silicon carbide, aluminumnitride, silicon nitride, and any combination thereof. More preferredare alumina, zirconia, titania, and any combination thereof.Particularly preferred are alumina and zirconia. Most preferred isalumina. The use of such a porous ceramic material facilitates theformation of a highly-densified LDH-containing separator layer.Preferred examples of the metal material include aluminum and zinc.Preferred examples of the polymer material include polystyrene,polyether sulfone, polypropylene, epoxy resins, polyphenylene sulfide,and any combination thereof. More preferably, a material having alkaliresistance (i.e., resistance to an electrolytic solution of a battery)is appropriately selected from among the preferred materials describedabove.

The porous substrate has an average pore size of preferably 0.001 to 1.5μm, more preferably 0.001 to 1.25 μm, still more preferably 0.001 to 1.0μm, particularly preferably 0.001 to 0.75 μm, most preferably 0.001 to0.5 μm. These ranges make it possible to form a dense LDH-containingseparator exhibiting water impermeability while ensuring desired waterpermeability in the porous substrate. In the present invention, theaverage pore size can be determined by measuring the largest length ofeach pore in an electron microscopic (SEM) image of the surface of theporous substrate. The magnification of the electron microscopic (SEM)image used in this measurement is 20,000 or more. All the measured poresizes are listed in order of size to calculate the average, from whichthe subsequent 15 larger sizes and the subsequent 15 smaller sizes,i.e., 30 diameters in total, are selected in one field of view. Theselected sizes of two fields of view are then averaged to yield theaverage pore size. The pore sizes can be measured by, for example, alength-measuring function of a SEM or image analysis software (e.g.,Photoshop manufactured by Adobe).

The surface of the porous substrate has a porosity of preferably 10 to60%, more preferably 15 to 55%, still more preferably 20 to 50%. Theseranges make it possible to form a dense LDH-containing separator layerthat exhibits water impermeability, while ensuring desired waterpermeability of the porous substrate. The surface porosity of the poroussubstrate is used in the present invention because it can be readilymeasured by image processing described below and substantially reflectsthe internal porosity of the porous substrate. Thus, if the surface ofthe porous substrate is dense, the inside of the porous substrate isalso dense. In the present invention, the porosity at the surface of theporous substrate can be measured by a method involving image processingas follows: 1) an electron microscopic (SEM) image of the surface of theporous substrate is taken at a magnification of 10,000 or more; 2) thegrayscale SEM image is read with an image analysis software, such asPhotoshop (manufactured by Adobe); 3) a monochromatic binary image isprepared by using tools named [image], [color compensation], and[binarization] in this order; and 4) the porosity (%) is calculated bydividing the number of pixels of the black area(s) by the number of allthe pixels of the image. Preferably, the porosity is measured over a 6μm×6 μm area of the surface of the porous substrate by image processing.More preferably, the porosities in three 6 μm×6 μm areas selected atrandom are averaged for objective evaluation.

The separator layer is formed on and/or in the porous substrate,preferably on the porous substrate. For example, the separator layer 20formed on the porous substrate 28 as illustrated in FIG. 4 is in theform of an LDH dense membrane, and the LDH dense membrane is typicallycomposed of LDH. The separator layer 20′ formed in the porous substrate28 as illustrated in FIG. 5 is typically composed of at least a portionof the porous substrate 28 and LDH because LDH is densely formed in theporous substrate 28 (typically in pores in the surface and its vicinityof the porous substrate 28). The separator layer 20′ illustrated in FIG.5 is prepared through removal of a membrane portion of the separatorlayer 20 illustrated in FIG. 4 by any known technique, such as polishingor machining.

The separator layer preferably exhibits water impermeability. Forexample, if water is brought into contact with one surface of theseparator layer at 25° C. for one week, water does not permeate theseparator layer. The separator layer composed of LDH is densified toexhibit water impermeability. If the dense membrane has local and/orincidental defects exhibiting water permeability, the defects may befilled with an appropriate repairing agent (e.g., an epoxy resin) forensuring water impermeability. Such a repairing agent does notnecessarily exhibit hydroxide ion conductivity. The surface of theseparator layer (typically LDH dense membrane) has a porosity ofpreferably 20% or less, more preferably 15% or less, still morepreferably 10% or less, particularly preferably 7% or less. A lowerporosity of the surface of the separator layer indicates a higherdensity of the separator layer (typically LDH dense membrane). Such ahigh density is preferred. The surface porosity of the separator layeris used in the present invention because it can be readily measured byimage processing described below and substantially reflects the internalporosity of the separator layer. Thus, if the surface of the separatorlayer is dense, the inside of the separator layer is also dense. In thepresent invention, the porosity of the surface of the separator layercan be measured by a method involving image processing as follows: 1) anelectron microscopic (SEM) image of the surface of the separator layeris taken at a magnification of 10,000 or more; 2) the grayscale SEMimage is read with image analysis software, such as Photoshop(manufactured by Adobe); 3) a monochromatic binary image is prepared byusing tools named [image], [color compensation], and [binarization] inthis order; and 4) the porosity (%) is calculated by dividing the numberof pixels of the black area(s) by the number of all the pixels of theimage. Preferably, the porosity is measured over a 6 μm×6 μm area of thesurface of the separator layer by image processing. More preferably, theporosities in three 6 μm×6 μm areas selected at random are averaged forobjective evaluation.

Preferably, the layered double hydroxide is composed of an aggregationof platy particles (i.e., platy LDH particles), and these platyparticles are oriented such that the tabular faces of the platyparticles are substantially perpendicular to or oblique to the surfaceof the porous substrate (i.e., the substrate surface). In particular,this preferred embodiment is applied to the case where the separatorlayer 20 is disposed in the form of an LDH dense membrane on the poroussubstrate 28 as illustrated in FIG. 4. Alternatively, this embodimentmay be applied to the case where LDH is densely formed in the poroussubstrate 28 (typically in pores in the surface and its vicinity of theporous substrate 28), and at least a portion of the porous substrate 28constitutes the separator layer 20′ as illustrated in FIG. 5.

As illustrated in FIG. 6, the LDH crystal is in the form of a platyparticle with a layered structure. The substantially perpendicular oroblique orientation described above is significantly beneficial for theLDH-containing separator layer (e.g., LDH dense membrane), because anoriented LDH-containing separator layer (e.g., an oriented LDH densemembrane) exhibits anisotropic hydroxide ion conductivity, i.e.,hydroxide ion conductivity along the orientation of the platy LDHparticles (i.e., parallel to layers of LDH) is much greater than thatperpendicular to the orientation of the platy LDH particles in theoriented LDH membrane. In fact, the present inventors have revealed thatthe hydroxide ion conductivity (S/cm) along the orientation of LDHparticles in an oriented LDH bulk body is an order of magnitude greaterthan the hydroxide ion conductivity (S/cm) perpendicular to theorientation of LDH particles. Thus, the substantially perpendicular oroblique orientation in the LDH-containing separator layer according tothe present invention fully or significantly leads to the anisotropichydroxide ion conductivity of the oriented LDH to the thicknessdirection of the layer (i.e., the direction perpendicular to the surfaceof the separator layer or the surface of the porous substrate), wherebythe conductivity in the thickness direction can be maximally orsignificantly increased. In addition, the LDH-containing separator layerhas a layered structure and thus exhibits lower resistance than an LDHbulk block. The LDH-containing separator layer having such anorientation readily conducts hydroxide ions in the thickness directionof the layer. Because of its high density, the LDH-containing separatorlayer is very suitable for use as a separator that requires highconductivity across the thickness of the layer and high density.

In a particularly preferred embodiment, the LDH-containing separatorlayer (typically LDH dense membrane) is composed of the platy LDHparticles highly oriented in the substantially perpendicular direction.If the platy LDH particles are highly orientated in the substantiallyperpendicular direction, the X-ray diffractometry of the surface of theseparator layer shows substantially no peak of (003) plane or a peak of(003) plane smaller than that of (012) plane (note: this shall not applyto the case where the porous substrate shows a peak at the same angle asthe peak of (012) plane of the platy LDH particles, because the peak of(012) plane of the platy LDH particles cannot be specified). Thischaracteristic peak profile indicates that the platy LDH particles ofthe separator layer are oriented substantially perpendicular to (i.e,perpendicular to or nearly perpendicular to, and preferablyperpendicular to) the separator layer. The peak of (003) plane isstrongest among peaks observed by X-ray diffractometry of non-orientedLDH powder. In contrast, the oriented LDH-containing separator layershows substantially no peak of (003) plane or the peak of (003) planesmaller than the peak of (012) plane because platy LDH particles areoriented substantially perpendicular to the separator layer. The reasonfor this is as follows: The c planes (001) including the (003) plane(where I is 3 or 6) are parallel to the layers of platy LDH particles.If the platy LDH particles are oriented substantially perpendicular tothe separator layer, the layers of platy LDH particles are alsosubstantially perpendicular to the separator layer and thus the X-raydiffractometry of the surface of the separator layer shows no peak orvery small peak of (001) plane (where I is 3 or 6). The peak of (003)plane, if present, tends to be stronger than the peak of (006) plane,and the use of the peak of (003) plane facilitates determination of thesubstantially perpendicular orientation as compared with the use of thepeak of (006) plane. Thus, the oriented LDH-containing separator layerpreferably shows substantially no peak of (003) plane or shows the peakof (003) plane smaller than the peak of (012) plane, which indicatesthat the highly perpendicular orientation is achieved.

The separator layer has a thickness of preferably 100 μm or less, morepreferably 75 μm or less, still more preferably 50 μm or less,particularly preferably 25 μm or less, most preferably 5 μm or less.Such a small thickness leads to a reduction in resistance of theseparator. The separator layer is preferably formed as an LDH densemembrane on the porous substrate. In this case, the thickness of theseparator layer corresponds to the thickness of the LDH dense membrane.If the separator layer is formed in the porous substrate, the thicknessof the separator layer corresponds to the thickness of a composite layercomposed of LDH and at least a portion of the porous substrate. If theseparator layer is formed on and in the porous substrate, the thicknessof the separator layer corresponds to the total thickness of the LDHdense membrane and the composite layer. The separator layer having theabove thickness exhibits a low resistance suitable for use in, forexample, a battery. The lower limit of the thickness of the oriented LDHmembrane, which may vary with the intended use of the membrane, may beany value. In order to ensure the hardness desirable for use in afunctional membrane, such as a separator, the thickness is preferably 1μm or more, more preferably 2 μm or more.

The LDH separator provided with the porous substrate is produced througha method involving (1) providing a porous substrate, (2) immersing theporous substrate in an aqueous stock solution containing magnesium ions(Mg²⁺) and aluminum ions (Al³⁺) in a total concentration of 0.20 to 0.40mol/L and further containing urea, and (3) hydrothermally treating theporous substrate in the aqueous stock solution, to form a separatorlayer containing a layered double hydroxide on and/or in the poroussubstrate.

(1) Provision of Porous Substrate

As described above, the porous substrate is preferably composed of atleast one selected from the group consisting of ceramic materials, metalmaterials, and polymer materials. The porous substrate is morepreferably composed of a ceramic material. Preferred examples of theceramic material include alumina, zirconia, titania, magnesia, spinel,calcia, cordierite, zeolite, mullite, ferrite, zinc oxide, siliconcarbide, aluminum nitride, silicon nitride, and any combination thereof.More preferred are alumina, zirconia, titania, and any combinationthereof. Particularly preferred are alumina and zirconia. Most preferredis alumina. The use of such a porous ceramic material facilitates theformation of a highly-densified LDH-containing separator layer. In thecase of a ceramic porous substrate, the porous substrate is preferablysubjected to, for example, ultrasonic cleaning or cleaning withion-exchange water.

In the case of a polymer porous substrate, the surfaces of the polymerare preferably anionized in advance. The anionized surface facilitatesthe nucleation of LDH at its anionic groups and also facilitates growthand the substantially perpendicular orientation of platy LDH particlesin subsequent steps. The polymer substrate having anionized surfaces canbe prepared by anionizing an anionizable polymer substrate by any knownmethod. Anionization is performed preferably by imparting at least oneof SO₃ ⁻ (sulfonation), OH⁻ (hydroxylation) and CO₂ ⁻ (carboxylation),which can be an anion in LDH, to the surface of a polymer substrate.Sulfonation is more preferred. The anionizable polymer substratepreferably has alkali resistance, i.e., resistance to an electrolytesolution of a battery. The anionizable polymer substrate is preferablycomposed of at least one selected from the group consisting ofpolystyrene, polyether sulfone, polypropylene, epoxy resins, andpolyphenylene sulfide, which are particularly suitable for sulfonation.Aromatic polymer substrates are particularly preferred because they arereadily anionized (particularly, sulfonated). Examples of the aromaticpolymer substrates include substrates composed of at least one selectedfrom the group consisting of polystyrene, polyether sulfone,polypropylene, an epoxy resin, and polyphenylene sulfide. Mostpreferably, the aromatic polymer substrate is composed of polystyrene.The sulfonation may be performed by soaking a sulfonatable polymersubstrate in an acid for sulfonating the sulfonatable polymer substrate,such as sulfuric acid (e.g., concentrated sulfuric acid), fumingsulfuric acid, chlorosulfuric acid, and sulfuric anhydride. Any othersulfonation techniques may also be used. The soaking in an acid for thesulfonation may be performed at room temperature or high temperature(e.g., 50 to 150° C.). The sulfonated aromatic polymer substrate shows atransmittance ratio T₁₆₀₁/T₁₁₂₇ of preferably not less than 0.920, morepreferably not less than 0.930, and more preferably not less than 0.940,where the ratio T₁₆₀₁/T₁₁₂₇ is calculated by dividing the transmittanceat 1601 cm⁻¹ (i.e., T₁₆₀₁) assigned to C═C stretching vibration of thephenyl group by the transmittance at 1127 cm⁻¹ (i.e., T₁₁₂₇) assigned tothe sulfonate group in the transmittance spectrum of the sulfonatedsurface of the aromatic polymer substrate measured by attenuated totalreflection (ATR) of Fourier transform infrared spectroscopy (FT-IR) onthe surface. In the transmittance spectrum, the absorption peak at 1601cm⁻¹ is assigned to C═C stretching vibration of the phenyl group, andthus the transmittance T₁₆₀₁ always has the same value, regardless ofthe presence or absence of the sulfonate group. In contrast, theabsorption peak at 1127 cm⁻¹ is assigned to the sulfonate group, andthus the transmittance T₁₁₂₇ has a lower value when the density of thesulfuric acid is higher. Thus, a higher T₁₆₀₁/T₁₁₂₇ indicates that moresulfonate groups are densely present on the surface of the polymersubstrate, and also indicates that nuclei of LDH that has taken thesulfonate groups as anions of an intermediate layer can be denselyformed, which contributes formation of the highly-densifiedLDH-containing functional layer. The ratio T₁₆₀₁/T₁₁₂₇ can have theabove-mentioned value by adjusting the soaking time in an acid for thesulfonation of the polymer substrate. For example, in the case of usingconcentrated sulfuric acid in the sulfonation, the soaking time ispreferably not less than 6 days, and more preferably not less than 12days. The anionized polymer substrate described above is preferablycleaned with ion-exchanged water and then dried at room temperature orhigh temperature (e.g., 30 to 50° C.).

(2) Immersion in Aqueous Stock Solution

The porous substrate is then immersed in the aqueous stock solution in adesired direction (e.g., horizontally or perpendicularly). Forhorizontal retention of the porous substrate, the porous substrate maybe hanged up in or suspended in a container of the aqueous stocksolution, or placed on the bottom of the container. For example, theporous substrate may be immobilized and suspended in the stock solutionand away from the bottom of the container. For perpendicular retentionof the porous substrate, a jig may be disposed that can hold the poroussubstrate perpendicularly to the bottom of the container. In any case, apreferred configuration or arrangement is one that can achieve growth ofLDH substantially perpendicular to the porous substrate (i.e., growth ofLDH such that the tabular faces of platy LDH particles are substantiallyperpendicular to or oblique to the surface of the porous substrate). Theaqueous stock solution contains magnesium ions (Mg²⁺) and aluminum ions(Al³⁺) in a specific total concentration and further contains urea. Ureais hydrolyzed into ammonia and raises the pH of the aqueous stocksolution, and metal ions co-existing in the aqueous stock solution formhydroxides, to produce LDH. The hydrolysis of urea, which also generatescarbon dioxide, can form LDH having carbonate ions as anions. Theaqueous stock solution contains magnesium ions (Mg²⁺) and aluminum ions(Al³⁺) in a total concentration of preferably 0.20 to 0.40 mol/L, morepreferably 0.22 to 0.38 mol/L, still more preferably 0.24 to 0.36 mol/L,particularly preferably 0.26 to 0.34 mol/L. Such a preferredconcentration range facilitates the nucleation and the crystal growth ina well-balanced manner and can form a highly-oriented, highly-densifiedLDH membrane. At a low total concentration of magnesium ions andaluminum ions, the crystal growth presumably dominates over thenucleation, resulting in a decrease in the number of LDH particles andan increase in size of the LDH particles. At a high total concentration,the nucleation presumably dominates over the crystal growth, resultingin an increase in the number of LDH particles and a decrease in size ofthe LDH particles.

Preferably, the aqueous stock solution contains dissolved magnesiumnitrate and aluminum nitrate; i.e., the aqueous stock solution containsnitrate ions besides magnesium ions and aluminum ions. In this case, themolar ratio of the urea to the nitrate ions (NO₃ ⁻) (i.e., urea/NO₃ ⁻)in the aqueous stock solution is preferably 2 to 6, more preferably 4 to5.

(3) Formation of LDH-Containing Separator Layer Through HydrothermalTreatment

The porous substrate is hydrothermally treated in the aqueous stocksolution to form the LDH-containing separator layer on and/or in theporous substrate. The hydrothermal treatment is performed in a hermeticcontainer at a temperature of preferably 60 to 150° C., more preferably65 to 120° C., still more preferably 65 to 100° C., particularlypreferably 70 to 90° C. The hydrothermal treatment temperature may haveany upper limit without causing thermal deformation of the poroussubstrate (e.g., a polymer substrate). The temperature can be elevatedat any rate in the hydrothermal treatment. The temperature elevationrate may be 10 to 200° C./h, and preferably 100 to 200° C./h, morepreferably 100 to 150° C./h. The time for the hydrothermal treatment maybe determined depending on the target density or thickness of the LDHmembrane.

After the hydrothermal treatment, the porous substrate is removed fromthe hermetic container, and then preferably cleaned with ion-exchangewater.

The LDH-containing separator layer in the LDH-containing compositematerial produced as described above is composed of densely assembledplaty LDH particles that are oriented in the substantially perpendiculardirection, which is beneficial for the conductivity. Thus, theLDH-containing separator layer is very suitable for use in a nickel-zincbattery that has suffered from growth of dendritic zinc which is anobstacle to practical use of this battery.

The above-described method may form LDH-containing separator layers onthe two surfaces of the porous substrate. Thus, in order to modify theLDH-containing composite material into a form suitable for use as aseparator, the LDH-containing separator layer on one surface of theporous substrate is preferably removed through mechanical scraping afterthe formation of the separator layers. Alternatively, it is desirable totake a measure to prevent formation of the LDH-containing separatorlayer on one surface of the porous substrate in advance.

Production Method of LDH Dense Plate

Preferred embodiments of the inorganic solid electrolyte in a plate forminclude a layered double hydroxide (LDH) dense body. The LDH dense bodyof the present invention may be prepared by any method, and onepreferable embodiment of the production method is described below. Thisproduction method is performed by compacting and firing a raw materialpowder of an LDH represented by hydrotalcite to obtain an oxide firedbody, allowing the oxide fired body to reproduce the layered doublehydroxide, and then removing excessive water. According to this method,a high-grade layered double hydroxide dense body having a relativedensity of 88% or greater can be provided and produced in a simple andstable manner.

(1) Provision of Raw Material Powder

A powder of a layered double hydroxide represented by general formula:M²⁺ _(1−x)M³⁺ _(x)(OH)₂A^(n−) _(x/n)·mH₂O (wherein M²⁺ is a divalentcation, M³⁺ is a trivalent cation, A^(n−) is an anion having a valencyof n, n is an integer of 1 or greater, x is 0.1 to 0.4, and m is anyreal number) is provided as a raw material powder. In the generalformula above, M²⁺ may be any divalent cation, and preferable examplesinclude Mg²⁺, Ca²⁺, and Zn²⁺, with Mg²⁺ being more preferable. M³⁺ maybe any trivalent cation, and preferable examples include Al³⁺ and Cr³⁺,with Al³⁺ being more preferable. A^(n−) may be any anion, and preferableexamples include OH⁻ and CO₃ ²⁻. Accordingly, it is preferable that inthe general formula above, at least M²⁺ comprises Mg²⁺, M³⁺ comprisesAl³⁺, and A^(n−) comprises OH⁻ and/or CO₃ ²⁻. The value of n is aninteger of 1 or greater and is preferably 1 or 2. The value of x is 0.1to 0.4 and is preferably 0.2 to 0.35. Such a raw material powder may bea commercially available layered double hydroxide product or may be araw material prepared by a known method such as liquid phase synthesistechnique using nitrate or chloride. The particle size of the rawmaterial powder is not limited as long as the desired layered doublehydroxide dense body can be obtained, and the volume-based D50 averageparticle diameter is preferably 0.1 to 1.0 μm and more preferably 0.3 to0.8 μm. This is because an excessively small particle diameter of theraw material powder is likely to result in aggregation of the powder,and it is highly possible that pores remain during compaction, while anexcessively large particle diameter results in poor compactability.

Optionally, the raw material powder may be calcined to obtain an oxidepowder. Although the calcination temperature at this stage is slightlydifferent depending on the constituting M²⁺ and M³⁺, the calcinationtemperature is preferably 500° C. or less and more preferably 380 to460° C., and calcination is performed in such a range that the particlediameter of the raw material does not largely change.

(2) Preparation of Compact

The raw material powder is compacted to obtain a compact. It ispreferable that this compaction is performed by, for example, pressingsuch that the compact after compaction and before firing (hereinafterreferred to as a compact) has a relative density of 43 to 65%, morepreferably 45 to 60%, and even more preferably 47% to 58%. The relativedensity of the compact can be determined by calculating the density fromthe size and weight of the compact and dividing the density by thetheoretical density, but since the weight of a compact is affected byadsorbed water, it is preferable to measure the relative density of acompact made from a raw material powder that has been stored for 24hours or longer in a desiccator at room temperature at a relativehumidity of 20% or less, or measure the relative density after storingthe compact under the foregoing conditions, in order to obtain a precisevalue. When a raw material powder that has been calcined to form anoxide powder is used, the relative density of the compact is preferably26 to 40% and more preferably 29 to 36%. In the case of using the oxidepowder, the relative density was determined by using a calculateddensity obtained in terms of a mixture of oxides as a denominator,assuming that the metal elements constituting the layered doublehydroxide had changed to their respective oxides due to calcination.Pressing, which is cited as an example, may be performed by metal-molduniaxial pressing or may be performed by cold isostatic pressing (CIP).In the case of cold isostatic pressing (CIP), it is preferable to use araw material powder that has been placed in a rubber container andvacuum-sealed or that has preliminarily compacted. In addition, the rawmaterial powder may be compacted by a known method such as slip castingor extrusion molding, and the compacting method is not particularlylimited. When a raw material powder that has been calcined to form anoxide powder is used, the compacting method is limited to drycompaction. The relative density of a compact from these methodsinfluences not only the strength of the resulting dense body but alsothe degree of orientation of layered double hydroxide particles thatusually have a plate shape, and it is therefore preferable to suitablyadjust the relative density within the aforementioned range at the stageof compaction in consideration of, for example, the application thereof.

(3) Firing Step

The compact obtained in the foregoing step is fired to obtain an oxidefired body. It is preferable that this firing is performed such that theoxide fired body has a weight that is 57 to 65% of the weight of thecompact and/or a volume that is 70 to 76% of the volume of the compact.When the weight is no less than 57% of the weight of the compact, aheterogeneous phase, from which a layered double hydroxide cannot bereproduced, is unlikely to be produced at the stage of reproduction ofthe layered double hydroxide, which is a subsequent step, and when theweight is no greater than 65%, firing is sufficient, and sufficientdensification is achieved in a subsequent step. Also, when the volume isno less than 70% of the volume of the compact, neither a heterogeneousphase nor cracks are likely to appear at the stage of reproducing alayered double hydroxide, which is a subsequent step, and when thevolume is no greater than 76%, firing is sufficient, and sufficientdensification is achieved in a subsequent step. When the raw materialpowder that has been calcined to form an oxide powder is used, it ispreferable to obtain an oxide fired body having a weight that is 85 to95% of the weight of the compact and/or a volume that is no less than90% of the volume of the compact. Irrespective of whether the rawmaterial powder is calcined or not, it is preferable that firing isperformed such that the oxide fired body has a relative density of 20 to40% in terms of oxide, more preferably 20 to 35%, and even morepreferably 20 to 30%. The relative density in terms of oxide isdetermined by using a calculated density obtained in terms of a mixtureof oxides as a denominator, assuming that the metal elementsconstituting the layered double hydroxide have changed to theirrespective oxides due to firing. A preferable firing temperature forobtaining an oxide fired body is 400 to 850° C., and more preferably 700to 800° C. It is preferable that the compact is retained at a firingtemperature within this range for 1 hour or longer, and a morepreferable retention time is 3 to 10 hours. In order to prevent thecompact from cracking due to the release of water and carbon dioxidecaused by rapid temperature increase, it is preferable to increase thetemperature to the aforementioned firing temperature at a rate of 100°C./h or less, more preferably 5 to 75° C./h, and even more preferably 10to 50° C./h. Accordingly, it is preferable to secure an overall firingtime from temperature increase to temperature decrease (100° C. or less)of 20 hours or longer, more preferably 30 to 70 hours, and even morepreferably 35 to 65 hours.

(4) Reproduction Step for Reproducing Layered Double Hydroxide

The oxide fired body obtained in the foregoing step is retained in orimmediately above an aqueous solution comprising the above-describedanion having a valency of n (A^(n−)) to reproduce a layered doublehydroxide, thereby providing a water-rich layered double hydroxidesolidified body. That is, the layered double hydroxide solidified bodyobtained by this production method inevitably contains excessive water.The anion contained in the aqueous solution may be the same anion as theanion contained in the raw material powder or may be a different anion.The retention of the oxide fired body in or immediately above theaqueous solution is preferably performed by a procedure of hydrothermalsynthesis in a closed vessel, and an example of such a closed vessel isa closed vessel made from Teflon (registered trademark), more preferablya closed vessel equipped with a jacket made from stainless steel or thelike. It is preferable that the formation of a layered double hydroxideis performed by retaining the oxide fired body at a temperature of 20°C. or greater and less than 200° C. in a state in which at least onesurface of the oxide fired body is in contact with the aqueous solution,a more preferable temperature is 50 to 180° C., and an even morepreferable temperature is 100 to 150° C. The oxide sintered body isretained at such a layered double hydroxide formation temperaturepreferably for 1 hour or longer, more preferably for 2 to 50 hours, andeven more preferably for 5 to 20 hours. Such a retention time makes itpossible to promote sufficient reproduction of a layered doublehydroxide and avoid or reduce a remaining heterogeneous phase. Anexcessively long retention time does not result in any particularproblem, and the retention time is suitably set in view of efficiency.

When carbon dioxide (carbonate ions) in air is intended to be used asthe anionic species of the aqueous solution comprising an anion having avalency of n used for the reproduction of a layered double hydroxide, itis possible to use ion exchanged water. When performing hydrothermaltreatment in a closed vessel, the oxide fired body may be immersed inthe aqueous solution, or treatment may be performed in such a state thatat least one surface is in contact with the aqueous solution by using ajig. In the case where treatment is performed in a state in which atleast one surface is in contact with the aqueous solution, the amount ofexcessive water is smaller than the amount required for completeimmersion, and therefore the subsequent step may be performed in ashorter period of time. However, an excessively small amount of theaqueous solution is likely to result in cracks, and it is preferable touse water in an amount greater than or equal to the weight of the firedbody.

(5) Dehydration Step

Excessive water is removed from the water-rich layered double hydroxidesolidified body obtained in the foregoing step. In this way, the layereddouble hydroxide dense body of the present invention is obtained. It ispreferable that this step of removing excessive water is performed in anenvironment having a temperature of 300° C. or less and an estimatedrelative humidity at the maximum temperature in the removal step of 25%or greater. In order to prevent rapid evaporation of water from thelayered double hydroxide solidified body, it is preferable to charge thesolidified body again into the closed vessel used in the reproductionstep for reproducing the layered double hydroxide and remove water, inthe case of dehydration at a temperature higher than room temperature. Apreferable temperature in this case is 50 to 250° C. and more preferably100 to 200° C. A more preferable relative humidity at the stage ofdehydration is 25 to 70% and even more preferably 40 to 60%. Dehydrationmay be performed at room temperature, and there is no problem as long asthe relative humidity in this case is within the range of 40 to 70% inan ordinary indoor environment.

EXAMPLES

The present invention will now be described in more detail by way ofExamples.

Example 1 Preparation and Evaluation of LDH Separator with PorousSubstrate

(1) Preparation of Porous Substrate

Boehmite (DISPAL 18N4-80, manufactured by Sasol Limited), methylcellulose, and ion-exchange water were weighed in proportions by mass of10:1:5, and were then kneaded together. The kneaded product wassubjected to extrusion molding with a hand press into a plate having asize sufficiently exceeding 5 cm×8 cm and a thickness of 0.5 cm. Theresultant green body was dried at 80° C. for 12 hours and then fired at1,150° C. for three hours, to prepare an alumina porous substrate. Theporous substrate was cut into a piece of 5 cm×8 cm.

The porosity at the surface of the resultant porous substrate wasdetermined by a method involving image processing. The porosity was24.6%. The porosity was determined as follows: 1) a scanning electronmicroscopic (SEM) image of the surface microstructure of the poroussubstrate was taken with a scanning electron microscope (SEM;JSM-6610LV, manufactured by JEOL Ltd.) (magnification: 10,000 or more)at an acceleration voltage of 10 to 20 kV; 2) the grayscale SEM imagewas read with image analysis software, such as Photoshop (manufacturedby Adobe); 3) a monochromatic binary image was prepared by using toolsnamed [image], [color compensation], and [binarization] in this order;and 4) the porosity (%) was determined by dividing the number of pixelsof the black areas by the number of all the pixels of the image. Theporosity was determined over a 6 μm×6 μm area of the surface of theporous substrate. FIG. 7 illustrates the SEM image of the surface of theporous substrate.

The average pore size of the porous substrate was about 0.1 μm. In thepresent invention, the average pore size was determined by measuring thelargest length of each pore in a scanning electron microscopic (SEM)image of the surface of the porous substrate. The magnification of thescanning electron microscopic (SEM) image used in this measurement was20,000. All the measured pore sizes were listed in order of size tocalculate the average, from which the subsequent 15 larger sizes and thesubsequent 15 smaller sizes, i.e., 30 sizes in total, were selected inone field of view. The selected sizes of two fields of view are thenaveraged to yield the average pore size. The pore sizes were measuredby, for example, a length-measuring function of SEM software.

(2) Cleaning of Porous Substrate

The resultant porous substrate was ultrasonically cleaned in acetone forfive minutes, in ethanol for two minutes, and then in ion-exchange waterfor one minute.

(3) Preparation of Aqueous Stock Solution

Magnesium nitrate hexahydrate (Mg(NO₃)₂.6H₂O, manufactured by KANTOCHEMICAL Co., Inc.), aluminum nitrate nonahydrate (Al(NO₃)₃.9H₂O,manufactured by KANTO CHEMICAL Co., Inc.), and urea ((NH₂)₂CO,manufactured by Sigma-Aldrich Corporation) were provided as rawmaterials for an aqueous stock solution. Magnesium nitrate hexahydrateand aluminum nitrate nonahydrate were weighed and placed in a beaker,and then ion-exchange water was added to the beaker to achieve a totalvolume of 75 mL, a ratio of the cations (Mg²⁺/Al³⁺) of 2, and a molarconcentration of the total metal ions (i.e., Mg²⁺ and Al³⁺) of 0.320mol/L. The resultant solution was agitated and urea was then added tothe solution. The added urea was weighed in advance to give a urea/NO₃ ⁻ratio of 4. The resultant solution was further agitated to prepare anaqueous stock solution.

(4) Formation of Membrane by Hydrothermal Treatment

The aqueous stock solution prepared in the above procedure (3) and theporous substrate cleaned in the above procedure (2) were enclosedtogether in a hermetic Teflon container (with an internal volume of 100mL and a stainless steel jacket). The porous substrate was horizontallysuspended and away from the bottom of the hermetic Teflon container suchthat the opposite surfaces of the porous substrate came into contactwith the aqueous stock solution. Thereafter, the porous substrate wassubjected to hydrothermal treatment at a hydrothermal temperature of 70°C. for 168 hours (7 days), to form oriented layered double hydroxidemembranes (separator layers) on the surfaces of the substrate. After theelapse of a predetermined period of time, the porous substrate wasremoved from the hermetic container, cleaned with ion-exchange water,and then dried at 70° C. for 10 hours, to form a dense membrane of thelayered double hydroxide (LDH) on the porous substrate (hereinafter thedense membrane will be referred to as “membrane sample”). The thicknessof the membrane sample was about 1.5 μm. A Layered doublehydroxide-containing composite material sample (hereinafter referred toas “composite material sample”) was thereby prepared. LDH membranes wereformed on the opposite surfaces of the porous substrate. In order to usethe composite material as a separator, the LDH membrane on one surfaceof the porous substrate was mechanically removed.

(5) Evaluations

(5a) Identification of Membrane Sample

A crystalline phase of a membrane sample was analyzed with an X-raydiffractometer (RINT-TTR III, manufactured by Rigaku Corporation) at avoltage of 50 kV, a current of 300 mA, and a measuring range of 10° to70°. The resultant XRD profile is illustrated in FIG. 8. The XRD profilewas compared with the diffraction peaks of a layered double hydroxide(or a hydrotalcite compound) described in JCPDS card No. 35-0964 foridentification of the membrane sample. The membrane sample wasidentified as a layered double hydroxide (LDH, or a hydrotalcitecompound). As shown in the XRD profile of FIG. 8, peaks derived fromalumina in the porous substrate on which the membrane sample was formed(i.e., the peaks marked with a circle) were also observed.

(5b) Observation of Microstructure

The surface microstructure of the membrane sample was observed with ascanning electron microscope (SEM; JSM-6610LV, manufactured by JEOLLtd.) at an acceleration voltage of 10 to 20 kV. FIG. 9 illustrates theresultant SEM image (i.e., a secondary electron image) of the surfacemicrostructure of the membrane sample.

A cross-section of the composite material sample was subjected to CPpolishing, and the microstructure of the polished cross-section wasobserved with a scanning electron microscope (SEM) at an accelerationvoltage of 10 to 20 kV. FIG. 10 illustrates the resultant SEM image ofthe microstructure of the polished cross-section of the compositematerial sample.

(5c) Measurement of Porosity

The porosity at the surface of the membrane sample was determined by amethod involving image processing. Specifically, the porosity wasdetermined as follows: 1) a scanning electron microscopic (SEM) image ofthe surface microstructure of the membrane was taken with a scanningelectron microscope (SEM; JSM-6610LV, manufactured by JEOL Ltd.)(magnification: 10,000 or more) at an acceleration voltage of 10 to 20kV; 2) the grayscale SEM image was read with image analysis software,such as Photoshop (manufactured by Adobe); 3) a monochromatic binaryimage was prepared by histogram thresholding with tools named [image],[color compensation], and [binarization] in this order; and 4) theporosity (%) was determined by dividing the number of pixels of theblack areas by the number of all the pixels of the image. The porositywas determined over a 6 μm×6 μm area of the surface of the membrane. Theporosity was 19.0%. This porosity was used to calculate the density D(hereinafter referred to as “membrane surface density”) of the surfaceof the membrane by the expression: D=100%−(the porosity at the surfaceof the membrane). The density D was 81.0%.

The porosity at the polished cross-section of the membrane sample wasalso determined. The porosity was determined as in the above procedureexcept for taking an electron microscopic (SEM) image of the polishedcross-section along the thickness of the membrane at a magnification of10,000 or more (through the above procedure (5b)). The determination ofthe porosity was performed on the cross-section of the membrane portionin the oriented membrane sample. The porosity at the polishedcross-section of the membrane sample was 3.5% on average (i.e., theaverage porosity of three polished cross-sections). The resultsdemonstrate a significantly high density of the membrane formed on theporous substrate.

(5d) Evaluation of Density

A density evaluation test was performed on the membrane sample fordetermining whether the sample has high density and thus waterimpermeability. With reference to FIG. 11A, a silicone rubber 122 havinga central opening 122 a (0.5 cm×0.5 cm) was bonded to the membranesample of composite material sample 120 prepared in (1) above (cut intoa piece of 1 cm×1 cm), and the resultant laminate was disposed betweentwo acrylic units 124 and 126 and bonded to these acrylic units. Theacrylic unit 124 disposed on the silicone rubber 122 has no bottom, andthus the silicone rubber 122 is bonded to the acrylic unit 124 such thatthe opening 122 a is exposed. The acrylic unit 126 disposed on theporous substrate side in view of composite material sample 120 has abottom and contains ion-exchange water 128. In this case, Al and/or Mgmay be dissolved in the ion-exchange water. Thus, these components arearranged to form an assembly such that the ion-exchange water 128 comesinto contact with the porous substrate of composite material sample 120if the assembly is inverted upside down. After formation of theassembly, the total weight thereof was measured. It goes without sayingthat the unit 126 has a closed vent (not shown) and the vent is openedafter inversion of the assembly. As illustrated in FIG. 11B, theassembly was inverted and left for one week at 25° C., and then thetotal weight thereof was measured again. Before measurement of theweight of the assembly, water droplets on the inner side(s) of theacrylic unit 124 were wiped off, if any. The density of the membranesample was evaluated on the basis of the difference between the totalweights of the assembly before and after the inversion. No change inweight of the ion-exchange water was observed even after the one-weektest at 25° C. The results demonstrate that the membrane sample (i.e.,functional membrane) exhibits high density and thus waterimpermeability.

Example 2 Production and Evaluation of Nickel-Zinc Battery

(1) Preparation of Separator with Porous Substrate

A separator provided with a porous substrate (hydrotalcite membrane onalumina substrate) (size: 5 cm×8 cm) was prepared as in Example 1.

(2) Preparation of Positive Electrode Plate

Particulate nickel hydroxide containing zinc and cobalt in the form ofsolid solution was prepared. The particulate nickel hydroxide was coatedwith cobalt hydroxide to yield a positive-electrode active material. Thepositive-electrode active material was mixed with a 2% aqueouscarboxymethyl cellulose solution to prepare a paste. The paste wasevenly applied to a current collector composed of a nickel poroussubstrate having a porosity of about 95% and dried so that the porosityof the positive-electrode active material was 50% to prepare a positiveelectrode plate having a region of 5 cm×5 cm coated with the activematerial. The amount of coating was adjusted so that the active materialcontained particulate nickel hydroxide corresponding to 4 Ah.

(3) Preparation of Negative Electrode Plate

A mixture of powdery zinc oxide (80 parts by weight), powdery zinc (20parts by weight), and particulate polytetrafluoroethylene (3 parts byweight) was applied onto a current collector composed of punched coppersheet, to prepare a negative electrode plate having a porosity of about50% and a region of 5 cm×5 cm coated with the active material. Theamount of coating was adjusted so that the active material containedpowdery zinc oxide corresponding to a positive electrode plate capacityof 4 Ah.

(4) Assembly of Battery

The positive electrode plate, the negative electrode plate, and theseparator provided with the porous substrate were assembled as describedbelow into a nickel-zinc battery illustrated in FIG. 1.

A rectangular parallelepiped casing composed of ABS resin and having nolid was provided. The separator provided with the porous substrate(hydrotalcite membrane on alumina substrate) was placed near the centerof the casing, and three edges of the separator were fixed to the innerwall of the casing with a commercially available resin adhesive. Thepositive electrode plate and the negative electrode plate were placed ina positive-electrode chamber and a negative-electrode chamber,respectively. The positive electrode plate and the negative electrodeplate were disposed so that a positive-electrode current collector and anegative-electrode current collector came into contact with the innerwall of the casing. A 6 mol/L aqueous KOH solution, serving as anelectrolytic solution, was injected into the positive-electrode chamberin an amount such that the region coated with the positive-electrodeactive material was completely submerged in the solution. The liquidlevel of the electrolytic solution in the positive-electrode chamber wasabout 5.2 cm from the bottom of the casing. A 6 mol/L aqueous KOHsolution, serving as an electrolytic solution, was injected into thenegative-electrode chamber in an amount such that the region coated withthe negative-electrode active material was completely submerged in thesolution. The amount of the electrolytic solution was adjusted so as tomeet the amount of water that will decrease during a charge mode. Theliquid level of the electrolytic solution in the negative-electrodechamber was about 6.5 cm from the bottom of the casing. The terminals ofthe positive-electrode current collector and the negative-electrodecurrent collector were connected to external terminals provided on a lidfor the casing. The casing lid was provided with a bypass tube forestablishing gas communication between the positive-electrode chamberand the negative-electrode chamber. Since the casing lid is disposed ata position which the electrolytic solution does not reach even after anincrease in amount of water through charging/discharging, the passage ofthe electrolytic solution through the bypass tube is essentiallyavoided. A gas-permeable water-repellent membrane may be disposed at anyposition of a gas communication segment of the bypass tube forpreventing the incidental intrusion or passage of water. The lid wasfixed to the casing by thermal fusion to hermetically seal the batterycasing, to produce a nickel-zinc battery including thepositive-electrode and negative-electrode chambers connected in gascommunication with each other by the bypass tube. In the battery, theseparator had a width of 5 cm and a height of 8 cm, and the region ofthe positive or negative electrode plates coated with the activematerial had a width of 5 cm and a height of 5 cm. Thus, an upper spaceof the positive-electrode or negative electrode chamber corresponding toa difference in height of 3 cm was respectively an extrapositive-electrode or negative-electrode space. The extrapositive-electrode space and the extra negative-electrode space wereconnected such that these spaces were in gas communication with eachother by the bypass tube disposed on the casing lid for preventing thepassage of the electrolytic solution through the bypass tube.

(5) Evaluation

The resultant nickel-zinc battery was subjected to constant-currentcharging for 10 hours (design capacity: 4 Ah, current: 0.4 mAcorresponding to 0.1 C). Neither the deformation of the casing nor theleakage of the electrolytic solution was observed after the charging.The liquid level of the electrolytic solution was observed after thecharging. The liquid level of the electrolytic solution in thepositive-electrode chamber was about 7.5 cm from the bottom of thecasing, and the liquid level of the electrolytic solution in thenegative-electrode chamber was about 5.2 cm from the bottom of thecasing. Although the amount of the electrolytic solution increased inthe positive-electrode chamber and the amount of the electrolyticsolution decreased in the negative-electrode chamber through thecharging, the region coated with the negative-electrode active materialwas immersed in a sufficient amount of the electrolytic solution. Thus,the electrolytic solution was retained in the casing in an amountsufficient for the charge/discharge reaction of the coatedpositive-electrode active material and negative-electrode activematerial through charge/discharge of the battery.

Example 3 Production and Evaluation of Nickel-Zinc Battery

A nickel-zinc battery was produced and evaluated as in Example 2, exceptthat an 8 mol/L aqueous KOH solution, serving as an electrolyticsolution, was injected into the positive-electrode chamber, and a 6mol/L aqueous KOH solution containing a saturated amount of ZnO (thuscontaining Zn(OH)₄ ²⁻), serving as an electrolytic solution, wasinjected into the negative-electrode chamber. The resultant nickel-zincbattery was subjected to charging as in Example 2. In a full chargestate, the KOH concentration in the positive-electrode chamber was about4.0 mol/L, and the KOH concentration in the negative-electrode chamberwas about 9 mol/L. Thus, the KOH concentration decreased in thepositive-electrode chamber through charging, whereas the KOHconcentration increased in the negative-electrode chamber throughcharging. Throughout the charge/discharge reaction, high conductivitywas maintained in the positive-electrode chamber on average, whereasrelatively high conductivity was maintained and ZnO was sufficientlydissolved in the negative-electrode electrolytic solution in thenegative-electrode chamber. During the charge/discharge mode of thebattery, the average KOH concentration was about 6 mol/L in thepositive-electrode chamber and about 7.5 mol/L in the negative-electrodechamber. In general, 1) ionic conductivity becomes maximum at a KOHconcentration of about 6 mol/L at 25° C. (see FIG. 12), and 2) anincrease in KOH concentration leads to an increase in ZnO solubility,and KOH is often used at a concentration of about 6 to 9 mol/L. Thus,the aqueous KOH solution in each of the positive-electrode andnegative-electrode chambers was maintained in a highly desirable statein view of ionic conductivity and ZnO solubility.

Example 4 Production and Evaluation of Nickel-Zinc Battery

A nickel-zinc battery was produced and evaluated as in Example 2, exceptthat an electrolytic solution containing no ZnO was injected into thenegative-electrode chamber, and the battery was subjected to thecharge/discharge reaction after sufficient dissolution of ZnO from thenegative electrode plate into the electrolytic solution. Favorableresults were obtained as in Example 2.

What is claimed is:
 1. A nickel-zinc battery comprising: a positiveelectrode comprising nickel hydroxide and/or nickel oxyhydroxide; apositive-electrode electrolytic solution comprising an alkali metalhydroxide, the positive electrode being immersed in thepositive-electrode electrolytic solution; a negative electrodecomprising zinc and/or zinc oxide; a negative-electrode electrolyticsolution comprising an alkali metal hydroxide, the negative electrodebeing immersed in the negative-electrode electrolytic solution; ahermetic container accommodating the positive electrode, thepositive-electrode electrolytic solution, the negative electrode, andthe negative-electrode electrolytic solution; and a separator exhibitinghydroxide ion conductivity and water impermeability, the separator beingdisposed in the hermetic container so as to separate apositive-electrode chamber accommodating the positive electrode and thepositive-electrode electrolytic solution from a negative-electrodechamber accommodating the negative electrode and the negative-electrodeelectrolytic solution, wherein the alkali metal hydroxide concentrationof the positive-electrode electrolytic solution differs from the alkalimetal hydroxide concentration of the negative-electrode electrolyticsolution.
 2. The nickel-zinc battery according to claim 1, wherein thealkali metal hydroxide concentration of the positive-electrodeelectrolytic solution is 1 to 9 M.
 3. The nickel-zinc battery accordingto claim 1, wherein the alkali metal hydroxide concentration of thenegative-electrode electrolytic solution is 3 to 18 M.
 4. Thenickel-zinc battery according to claim 1, wherein the alkali metalhydroxide concentration of the positive-electrode electrolytic solutionis lower than the alkali metal hydroxide concentration of thenegative-electrode electrolytic solution in a full charge state.
 5. Thenickel-zinc battery according to claim 1, wherein the alkali metalhydroxide concentration of the positive-electrode electrolytic solutionis higher than the alkali metal hydroxide concentration of thenegative-electrode electrolytic solution in a discharge end state. 6.The nickel-zinc battery according to claim 1, wherein the nickel-zincbattery is prepared in a discharge end state, and the alkali metalhydroxide concentration of the positive-electrode electrolytic solutionis higher than the alkali metal hydroxide concentration of thenegative-electrode electrolytic solution during preparation of thebattery.
 7. The nickel-zinc battery according to claim 1, wherein thenickel-zinc battery is prepared in a full charge state, and the alkalimetal hydroxide concentration of the positive-electrode electrolyticsolution is lower than the alkali metal hydroxide concentration of thenegative-electrode electrolytic solution during preparation of thebattery.
 8. The nickel-zinc battery according to claim 1, wherein theaverage alkali metal hydroxide concentration of the negative-electrodeelectrolytic solution is higher than the average alkali metal hydroxideconcentration of the positive-electrode electrolytic solution from afull charge state to a discharge end state or from a discharge end stateto a full charge state.
 9. The nickel-zinc battery according to claim 1,wherein the alkali metal hydroxide is potassium hydroxide.
 10. Thenickel-zinc battery according to claim 1, wherein the positive-electrodeelectrolytic solution is an aqueous potassium hydroxide solution, andthe negative-electrode electrolytic solution is an aqueous potassiumhydroxide solution containing Zn(OH)₄ ²⁻.
 11. The nickel-zinc batteryaccording to claim 1, wherein the positive-electrode chamber has anextra positive-electrode space having a volume that meets a variation inamount of water in association with reaction at the positive electrodeduring charge and discharge of the battery, and the negative-electrodechamber has an extra negative-electrode space having a volume that meetsa variation in amount of water in association with reaction at thenegative electrode during charge and discharge of the battery.
 12. Thenickel-zinc battery according to claim 11, wherein the extrapositive-electrode space has a volume greater than the amount of waterthat will increase in association with reaction at the positiveelectrode during the charge of the battery; the extra positive-electrodespace is not preliminarily filled with the positive-electrodeelectrolytic solution; the extra negative-electrode space has a volumegreater than the amount of water that will decrease in association withreaction at the negative electrode during the charge of the battery; andthe extra negative-electrode space is preliminarily filled with anamount of the negative-electrode electrolytic solution that willdecrease during the charge of the battery.
 13. The nickel-zinc batteryaccording to claim 11, wherein the extra positive-electrode space has avolume greater than the amount of water that will decrease inassociation with reaction at the positive electrode during the dischargeof the battery; the extra positive-electrode space is preliminarilyfilled with an amount of the positive-electrode electrolytic solutionthat will decrease during the discharge of the battery; the extranegative-electrode space has a volume greater than the amount of waterthat will increase in association with reaction at the negativeelectrode during the discharge of the battery; and the extranegative-electrode space is not preliminarily filled with thenegative-electrode electrolytic solution.
 14. The nickel-zinc batteryaccording to claim 1, wherein the extra positive-electrode space is notfilled with the positive electrode and the extra negative-electrodespace is not filled with the negative electrode.
 15. The nickel-zincbattery according to claim 11, further comprising a gas flow channelthat connects the extra positive-electrode space to the extranegative-electrode space such that the spaces are in gas communicationwith each other.
 16. The nickel-zinc battery according to claim 11,wherein the separator is vertically disposed, the extrapositive-electrode space is provided in an upper portion of thepositive-electrode chamber, and the extra negative-electrode space isprovided in an upper portion of the negative-electrode chamber.
 17. Thenickel-zinc battery according to claim 1, wherein the separatorcomprises an inorganic solid electrolyte.
 18. The nickel-zinc batteryaccording to claim 17, wherein the inorganic solid electrolyte has arelative density of 90% or more.
 19. The nickel-zinc battery accordingto claim 17, wherein the inorganic solid electrolyte comprises a layereddouble hydroxide.
 20. The nickel-zinc battery according to claim 19,wherein the layered double hydroxide has a basic composition representedby the general formula:M²⁺ _(1−x)M³⁺ _(x)(OH)₂A^(n−) _(x/n) .mH₂O where M²⁺ represents at leastone divalent cation, M³⁺ represents at least one trivalent cation,A^(n−) represents an n-valent anion, n is an integer of 1 or more, x is0.1 to 0.4, and m is any real number.
 21. The nickel-zinc batteryaccording to claim 17, wherein the inorganic solid electrolyte is in aplate, membrane, or layer form.
 22. The nickel-zinc battery according toclaim 1, further comprising a porous substrate on either or both of thesurfaces of the separator.
 23. The nickel-zinc battery according toclaim 22, wherein the inorganic solid electrolyte is in a membrane orlayer form, and is disposed on or in the porous substrate.
 24. Thenickel-zinc battery according to claim 17, wherein the inorganic solidelectrolyte is densified through hydrothermal treatment.
 25. Thenickel-zinc battery according to claim 1, further comprising apositive-electrode collector in contact with the positive electrode, anda negative-electrode collector in contact with the negative electrode.26. The nickel-zinc battery according to claim 22, wherein the inorganicsolid electrolyte comprises a layered double hydroxide, wherein thelayered double hydroxide is composed of an aggregation of platyparticles, and wherein the platy particles are oriented such thattabular faces of the platy particles are perpendicular to or oblique tothe surface of the porous substrate.
 27. The nickel-zinc batteryaccording to claim 22, wherein the porous substrate is composed of apolymer, and the polymer is at least one selected from the groupconsisting of polystyrene, polyether sulfone, polypropylene, epoxyresin, and polyphenylene sulfide.